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robertb answered: 4011 problems
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Matrices-and-determiminant/391615: find the determinant of [x y]
[z w]
1 solutions

Answer 277869 by robertb(4012) About Me  on 2011-01-07 03:18:00 (Show Source):


Trigonometry-basics/391582: How do you solve for x in cos2x = cosx, -pi<=x<=pi?
1 solutions

Answer 277868 by robertb(4012) About Me  on 2011-01-07 03:16:32 (Show Source):
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cos+2x+=+cos+x <==> 2%28cos+x%29%5E2+-+1+=+cosx <==> 2%28cos+x%29%5E2+-+cosx+-+1+=+0 <==> (2 cosx + 1)(cosx - 1) = 0 ==> cosx = -1/2 or cos x = 1.
From cosx = -1/2, x = -2pi%2F3,+2pi%2F3.
From cos x = 1, x = 0.
The solutions are then 0,-2pi%2F3,+2pi%2F3.


Percentage-and-ratio-word-problems/391596: you purchase a desk for 60% of the original price p. the price is $32 less then the original price.write and solve an equation to find the original price of the desk.
1 solutions

Answer 277864 by robertb(4012) About Me  on 2011-01-07 03:02:14 (Show Source):
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0.60p = p - 32
==> -0.40p = -32
==> p = -32/-0.40 = $80, the original price of the desk.


Trigonometry-basics/391597: How do I approach the following question: If x sin A + y cos A = p and x cos A - y sin A = q, find x square + y square.
1 solutions

Answer 277863 by robertb(4012) About Me  on 2011-01-07 02:57:16 (Show Source):
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= q cos A - p sin A
= p cos A - q sin A
after using Cramer's rule to solve for x and y.
Then
x%5E2+%2B+y%5E2
=
=
=p%5E2+%2B+q%5E2+-+2pq+sin+2A.


Quadratic_Equations/391624: Determine whether each pair of functions are inverse functions. f(x)=-4x+1, g(x)=1/4(1-x)
1 solutions

Answer 277861 by robertb(4012) About Me  on 2011-01-07 02:38:59 (Show Source):
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f%28x%29+=+-4x+%2B+1, and g%28x%29+=+%281%2F4%29%2A%281-x%29. They are inverses, because
%28fog%29%28x%29+=+-4%2A%28%281%2F4%29%2A%281-x%29%29+%2B+1+=+-1+%2B+x+%2B+1+=+x, and
%28gof%29%28x%29+=+%281%2F4%29%2A%281-%28-4x+%2B+1+%29%29+=+%281%2F4%29%2A%284x+%29+=+x.


Quadratic_Equations/391626: Determine whether each pair of functions are inverse functions. f(x)=2x, g(x)=-2x
1 solutions

Answer 277860 by robertb(4012) About Me  on 2011-01-07 02:32:02 (Show Source):
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No, not inverse functions, because fog = -4x, and not equal to x. Similarly gof = -4x, and not equal to x.


Probability-and-statistics/391657: Suppose the variable X has a normal distribution with mean 36 inches and standard deviation 2 inches.What proportion of these values is between 32 and 40 inches? If one of these X values is chosen at random, what is the probability that it is between 34 and 38 inches?

1 solutions

Answer 277858 by robertb(4012) About Me  on 2011-01-07 02:22:46 (Show Source):
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The values between 32 and 40 inches are within 2 standard deviations from the mean 36, and so covers around 95.4% of the distribution. The values between 34 and 38 inches are within 1 standard deviation from the mean 36, and so covers around 68.2% of the distribution.


Travel_Word_Problems/391667: You live 50 miles away from mr.jones. You both leave home at 5 and drive toward each other. mr. jones travels at 35 mph and you travel at 40 mph. At what time will you pass mr. jones on the road?
1 solutions

Answer 277857 by robertb(4012) About Me  on 2011-01-07 02:14:35 (Show Source):
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35t + 40t = 50 ==> 75t = 50 ==> t = 2/3 hr, or 40 minutes. You and Mr. Jones will meet at 5:40.


Rectangles/391117: If you double all of the sides of a square by what percentage has the area increased?
1 solutions

Answer 277624 by robertb(4012) About Me  on 2011-01-06 03:53:18 (Show Source):
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original area = s%5E2
new area = %282s%29%5E2+=+4s%5E2
==> percentage increase is %28%284s%5E2+-+s%5E2%29%2Fs%5E2%29%2A100+=+300%.
You stated: "When we divide a whole number by a FRACTION [caps mine], our answer is larger than we started with. Explain why."
In fact, this is true only if the dividing FRACTION is between 0 and 1 exclusive, and the number we're dividing is positive.
Let a, b be positive integers, and let a > b, so that b/a < 1.
Then c%2F%28b%2Fa%29+=+c%2Aa%2Fb+%3E+c by a multiplicative property of inequality.


Inequalities/391120: Eight less than a number is no more than 14 and no less than 5
1 solutions

Answer 277623 by robertb(4012) About Me  on 2011-01-06 03:49:27 (Show Source):


Travel_Word_Problems/391311: A train travels 80 miles per hour. A plane travels 200 miles per hour. How many hours longer will it take the train to travel 1,600 miles than it will take the plane?
1 solutions

Answer 277622 by robertb(4012) About Me  on 2011-01-06 03:42:53 (Show Source):
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1600%2F80+-+1600%2F200+=+20-+8+=+12 hours longer.


test/389908: Find the x and y coordinates of the points on x^2 + y^2=16 at which the tangent line(s) are:
a) horizontal b) have a slope of 1
1 solutions

Answer 277620 by robertb(4012) About Me  on 2011-01-06 02:39:25 (Show Source):
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a) The tangent lines are horizontal at the points (0,4) and (0,-4).
b) The tangent lines have slope 1 at the intersection points of the circle with the line y = -x. Solving:
x%5E2+%2B+y%5E2+=+x%5E2+%2B+%28-x%29%5E2+=+2x%5E2+=+16 ==> x%5E2+=+8, or
x+=+2sqrt%282%29, x+=++-2sqrt%282%29.
Then the points are (2sqrt%282%29,+-2sqrt%282%29) and (-2sqrt%282%29,+2sqrt%282%29).


Probability-and-statistics/391102: Rebecca rolled a fair number cube twice. What is the probability that she will roll a number divisible by 3 on both rolls?
1 solutions

Answer 277619 by robertb(4012) About Me  on 2011-01-06 02:30:43 (Show Source):
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(1/3)*(1/3) = 1/9


test/389816: Growth and decay.
In march, yoda invested $1500 into the stock market. In December his stock portfolio was worth $5200. What was the monthly rate of change in his portfolio?
1 solutions

Answer 277618 by robertb(4012) About Me  on 2011-01-06 02:27:10 (Show Source):
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I will assume that YOU invested $1500 AT THE BEGINNING of the month of March, and that at the END of December, YOUR stock portfolio has worth $5200. So that's after 10 months. YOUR portfolio's monthly rate of change r is then computed as follows:
5200+=+1500%281+%2Br%29%5E10
==> r++=+root%2810%2C+5200%2F1500%29+-+1+=+0.1324, to 4 decimal places.
That is 13.24% per month.


Functions/391212: For g(x)=4x-9, find g(x) when x=3 and when x=5

1 solutions

Answer 277617 by robertb(4012) About Me  on 2011-01-06 02:08:06 (Show Source):
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g(3) = 4*3 - 9 = 12 - 9 = 3,
g(5) = 4*5 - 9 = 20 - 9 = 11.


Trigonometry-basics/391223: Solve the following equation for x:
sec x =1, for the interval[0,pi]?
1 solutions

Answer 277616 by robertb(4012) About Me  on 2011-01-06 02:04:57 (Show Source):
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sec x = 1 ==> cos x = 1. The only x-value satisfying the last equation in [0,pi] is x = 0.


logarithm/391251: If f(g(x))=e^.05x, write formulas for f(x) and g(x).
1 solutions

Answer 277614 by robertb(4012) About Me  on 2011-01-06 02:02:21 (Show Source):
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g(x) could be 0.05x, while f(x) could be e%5Ex.


test/391281: What is the least number that when divided by 7 has a remainder of 4, and when divided by 8 has a remainder of 5, and when divided by 9 has a remainder of 6?
1 solutions

Answer 277613 by robertb(4012) About Me  on 2011-01-06 02:00:14 (Show Source):
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I will assume that the least number you're asking for is a positive number.
Then we have to find the smallest positive integer n that satisfies the simultaneous system of congruences
n = 4 mod 7,
n = 5 mod 8,
n = 6 mod 9.
We use the Chinese Remainder Theorem using the extended Euclidean algorithm. (For the statement of the theorem and symbolism go to http://en.wikipedia.org/wiki/Chinese_remainder_theorem).
(31)(7) + (-3)(8*9) = 31*7 + -3*72 = 1 ==> e1 = -216 (=-3*72)
8*8 + -1*(7*9) = 8*8 + -1*63 = 1 ==>e2 = -63 (=-1*63), and
25*9 + -4*(7*8) = 25*9 + -4*56 = 1 ==> e3 = -224. ( = -4*56)
Then the solution n is the linear combination n = 4*e1 + 5*e2 + 6*e3 = 4*-216 +5*-63 +6*-224 = -2523 mod 504, where 504 = 7*8*9.
The least positive integer satisfying n = -2523 mod 504 is 501.
Therefore the final answer is 501.


Triangles/389893: The vertices of ABC are A(-8, 0), B(0, 0), and C(0, -4). If a circle is circumscribed around ABC, where would the center of the circle be located?
1 solutions

Answer 277551 by robertb(4012) About Me  on 2011-01-05 22:13:16 (Show Source):
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The circumscribing circle must have center (-4, -2), which is actually the midpoint of the segment AC.


Trigonometry-basics/391236: If A and B are two angles in Quadrant 2 such that tan A=-1/2 and tan B=-2/3, find cos(A+B).
Thank you so much for all of your help!!!!
God Bless!!!!!
1 solutions

Answer 277545 by robertb(4012) About Me  on 2011-01-05 21:56:04 (Show Source):
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tan A = -1/2 ==> 1+%2B+%28tan+A%29%5E2++=+1%2B1%2F4+=+5%2F4+=+%28sec+A%29%5E2+ ==> sec+A+=+-sqrt%285%29%2F2 ==> cos+A+=+-2%2Fsqrt%285%29.
tan B = -2/3 ==> 1+%2B+%28tan+B%29%5E2++=+1%2B4%2F9+=+13%2F9+=+%28sec+B%29%5E2+ ==> sec+B+=+-sqrt%2813%29%2F3 ==> cos+B+=+-3%2Fsqrt%2813%29.
Now


Complex_Numbers/389874: please solve algebraically for x:
square root (3x+1)+1=x
1 solutions

Answer 277539 by robertb(4012) About Me  on 2011-01-05 21:44:05 (Show Source):
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sqrt%283x%2B1%29%2B1=x,
==> sqrt%283x%2B1%29+=+x+-+1,
==> 3x%2B1+=+x%5E2+-+2x+%2B+1,after squaring both sides.
==> 0+=+x%5E2+-+5x, or x(x - 5) = 0.
==> x = 0 or x = 5. The 1st x-value will not satisfy the original equation, and the 2nd x-value does. Therefore final answer is x = 5.


logarithm/391133: Can you help me solve this? I have a really hard time with Logarithms: log(2x+7) base 4 = log(81) base 3
1 solutions

Answer 277528 by robertb(4012) About Me  on 2011-01-05 21:36:48 (Show Source):
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log(2x+7) base 4 = log(81) base 3
<==> log(2x+7) base 4 = 4
<==> 4%5E4+=+2x%2B7
==> 2x + 7 = 256,
x = 249/2.


Functions/390984: Let f(x) = sqrt of x and g(x)= 2x+5. Determine the domain of the composite function and then find the composite function.
(g*f)(x)
Domain: _________
(g*f)(x)= _________
1 solutions

Answer 277521 by robertb(4012) About Me  on 2011-01-05 21:33:15 (Show Source):
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(gof)(x) = g(f(x)) = g(sqrt%28x%29) = 2sqrt%28x%29 + 5.
The domain of gof is [0, infinity).


Trigonometry-basics/391225: Determine the range of y = tan 2x
1 solutions

Answer 277513 by robertb(4012) About Me  on 2011-01-05 21:27:48 (Show Source):
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The range of tan(2x) is still (-infinity, infinity). What changes is its period. From a period of pi (from tanx), the period becomes pi%2F2.


Triangles/391156: in equilateral triangle one angle measures 3x+y the other 2x+40 and the third 2y what is the value of x and what is the value of y
1 solutions

Answer 277506 by robertb(4012) About Me  on 2011-01-05 21:19:46 (Show Source):
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3x+y = 60
2x+ 40 = 60
2y = 60, because equilateral <==> equiangular.
Then y = 30 degrees, and x = 10 degrees.


Trigonometry-basics/391183: Which expression is equivalent to log7 2401 - log3 243
1 solutions

Answer 277502 by robertb(4012) About Me  on 2011-01-05 21:11:22 (Show Source):
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2401+=+7%5E4 and 243+=+3%5E5, thus log7 2401 - log3 243 = 4 - 5 = -1.


Miscellaneous_Word_Problems/391200: On a scale drawing, 48 feet is represented by 12 inches.How many feet does a scale 1/4 inch represent?

1 solutions

Answer 277499 by robertb(4012) About Me  on 2011-01-05 21:07:48 (Show Source):
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48%2Fx+=+12%2F0.25 <==> 48%2Fx+=+48, so x = 1 foot.


Inequalities/391216: solve 5x+3x>-8
1 solutions

Answer 277495 by robertb(4012) About Me  on 2011-01-05 21:05:22 (Show Source):
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5x+3x>-8
==>8x > -8
==>x > -1 after division by 8, a positive number.
In interval notation , (-1, -infinity).


Rational-functions/391155: can you please help me work out this problem. i need to find the range of this function: {{3+x^2/ 4-x^2}}. i tried writing the problem as two binomials on top and two binomials on the bottom and solve for x but i am not sure if that is what i am suppose to do. so can you please help me solve this problem.
Thank you so much for your time.
1 solutions

Answer 277492 by robertb(4012) About Me  on 2011-01-05 21:03:14 (Show Source):
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I will assume that your function is y+=+%283%2Bx%5E2%29%2F%284-x%5E2%29.
To find the range solve for x in terms of y and then state conditions on y that may arise.
y+=+%283%2Bx%5E2%29%2F%284-x%5E2%29
<==> 4y+-+yx%5E2+=+3+%2B+x%5E2
<==> 0+=+%281%2By%29x%5E2+-+4y+%2B+3
==> , using the quadratic formula.
Since the values of x must be real, the conditions we should impose on y are
+%281%2By%29%2A%283-4y%29%3E=+0 and y is not equal to -1.
The solution set to this inequality is the union of intervals (-infinity, -1)U[3/4, infinity). This is the RANGE of the function.


Travel_Word_Problems/390922: A car travels y miles in t hours, what is its speed?
1 solutions

Answer 277260 by robertb(4012) About Me  on 2011-01-05 05:05:30 (Show Source):
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y/t miles per hour average speed


Travel_Word_Problems/390906: The distance from a star to a planet is 9.1 X 10^18 m. How long does it take light, traveling at 10^16 m/year, to travel from the star to the planet?
1 solutions

Answer 277259 by robertb(4012) About Me  on 2011-01-05 05:04:23 (Show Source):
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%28%289.1%2910%5E18%29%2F10%5E16+=+%289.1%29+10%5E2 years, or 910 years.