SOLUTION: solve for x: 7x^2 - 21x - 3 = 0

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Question 937693: solve for x:
7x^2 - 21x - 3 = 0

Found 3 solutions by stanbon, TimothyLamb, josgarithmetic:
Answer by stanbon(75887) About Me  (Show Source):
You can put this solution on YOUR website!
The problem I would like to solve is:
7x^2 - 21x - 3 = 0
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You are correct ; the AC Method does not work on your problem.
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Use the Quadratic Formula::
x = [-b +- sqrt(b^2-4ac)]/(2a)
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x = [21 +- sqrt(441-4*7*-3)]/14
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x = [21 +- sqrt(525)]/14
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x = (21+5sqrt(5))/14 or x = (21-5sqrt(5))/14
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Cheers,
Stan H.
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Answer by TimothyLamb(4379) About Me  (Show Source):
You can put this solution on YOUR website!
7x^2 - 21x - 3 = 0
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the above quadratic equation is in standard form, with a=7, b=-21 and c=-3
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to solve the quadratic equation, by using the quadratic formula, copy and paste this:
7 -21 -3
into this solver: https://sooeet.com/math/quadratic-equation-solver.php
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the quadratic has two real roots at:
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x = 3.13663418
x = -0.1366342
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now solved by completing the square:
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7xx - 21x - 3 = 0
7xx - 21x = 3
(7xx - 21x = 3)/7
xx - 3x = 3/7
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complete the square:
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xx - 3x + (3/2)(3/2) = (3/7) + (3/2)(3/2)
(x - 3/2)^2 = (3/7) + (9/4)
(x - 3/2)^2 = (3*4/28) + (9*7/28)
(x - 3/2)^2 = (75/28)
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solve for x:
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sqrt( (x - 3/2)^2 ) = sqrt(75/28)
x - 3/2 = sqrt(75/28)
x = (+ or -) sqrt(75/28) + 3/2
x = +sqrt(75/28) + 3/2
x = -sqrt(75/28) + 3/2
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the quadratic has two real roots at:
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x = 3.13663418
x = -0.1366342
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Answer by josgarithmetic(39617) About Me  (Show Source):
You can put this solution on YOUR website!
Can you try to factorize the left side?

%287x%2B-+1%29%28x%2B-+%28-3%29%29, or %287x%2B-+3%29%28x%2B-+1%29, four possibilities. Does one of them work?

(7x+1)(x-3)=....1x-21x... NO.
(7x-1)(x+3)=......%2821-1%29x.... NO.

(7x+3)(x-1)...........(7-3)x....... NO

Last combination will not work either.

You will need to use the formula for the general solution of a quadratic equation.


Follow this formula:
ax%5E2%2Bbx%2Bc=0;
x=%28-b%2B-+sqrt%28b%5E2-4ac%29%29%2F%282a%29