An algebra problem by Phong Huỳnh Phú

Algebra Level 3

7 x + 7 + 7 x 6 + 2 49 x 2 + 7 x 42 = 181 14 x \sqrt{ 7x + 7 } + \sqrt{ 7x - 6 } + 2 \sqrt{ 49x^2 + 7x - 42 } = 181 - 14x

Solve for x x .


The answer is 6.

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1 solution

Noel Lo
Jul 19, 2015

Hello, like what the title said, do not be fooled into thinking that you can factorise the left hand side of the equation to a^2+2ab+b^2.

7x+7+2(49x^2+7x-42)^(1/2) + 7x-6 = 182 - (7x+7)^(1/2) - (7x-6)^(1/2)

Then ((7x+7)^(1/2) + (7x-6)^(1/2))^2 = 182 - (7x+7)^(1/2) - (7x-6)^(1/2)

Let a = (7x+7)^(1/2) + (7x-6)^(1/2) so that a^2 = 182 - a. Solving we get a=-14 or 13 but we reject the negative solution as square roots are always positive.

Solving for x we get x=6.

Speechless

Thanos TheMadTitan - 4 years, 5 months ago

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