SOLUTION: 1) The line y= 3x + 4 is a tangent to the curve y=ax3+bx2 +2x+7 at the point (1,7) 1a.) Determine the values of a and b 1b. ) The tangent to the curve is parallel to y= 3x +4 a

Algebra ->  Test -> SOLUTION: 1) The line y= 3x + 4 is a tangent to the curve y=ax3+bx2 +2x+7 at the point (1,7) 1a.) Determine the values of a and b 1b. ) The tangent to the curve is parallel to y= 3x +4 a      Log On


   



Question 1137358: 1) The line y= 3x + 4 is a tangent to the curve y=ax3+bx2 +2x+7 at the point (1,7)
1a.) Determine the values of a and b
1b. ) The tangent to the curve is parallel to y= 3x +4 at one additional point. Determine the x-coordinate of this point

Answer by MathLover1(20850) About Me  (Show Source):
You can put this solution on YOUR website!

1) The line y=+3x+%2B+4 is a tangent to the curve y=ax%5E3%2Bbx%5E2+%2B2x%2B7+at the point (1,7)
1a.) Determine the values of a and b+
use given point (1,7) and plug coordinates in y=ax%5E3%2Bbx%5E2+%2B2x%2B7+
in this case, the curve must go through (1,7)
7=a%2A1%5E3%2Bb%2A1%5E2+%2B2%2A1%2B7
7=a%2Bb+%2B2%2B7
a%2Bb=7-9
a%2Bb=-2....solve for a
a=-2-b.....eq.1

and the derivative of y=ax%5E3%2Bbx%5E2+%2B2x%2B7 is
y'=3ax%5E2%2B2bx+%2B2
at this point must be 3 (the slope of the tangent):
3a+%2B+2b+%2B+2+=+3
3a+%2B+2b=3-+2+...solve for a
3a+=1-2b+
a+=1%2F3-2b%2F3+.......eq.2
from eq.1 and eq.2 we have
-2-b=1%2F3-2b%2F3...solve for b
2b%2F3-b=1%2F3%2B2....both sides multiply by 3
2b-3b=1%2B6
-b=7
b=-7
go to eq.1
a=-2-b.....eq.1 substitute b
a=-2-%28-7%29
a=-2%2B7
a=+5
=>
y=5x%5E3-7x%5E2+%2B2x%2B7+

1b. ) The tangent to the curve is parallel to y=+3x+%2B4 at one additional point. Determine the x-coordinate of this point.
For part b, you will need to solve a quadratic using the values of a and b to find the point where the derivative is again = 3.
y'=3%2A5x%5E2%2B2%28-7%29x+%2B2
3=15x%5E2-14x+%2B2
15x%5E2-14x+%2B2-3=0
15x%5E2-14x+-1=0
15x%5E2-15x%2Bx+-1=0
15x%28x-1%29%2B%28x+-1%29=0
%28x+-+1%29+%2815x+%2B+1%29+=+0
x=1-> already given in point (1,7)
+15x+%2B+1=+0->x=-1%2F15->the x-coordinate of this point