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Algebra Level 4

Find the sum of the solutions of the equation: x 3 + 2 x 2 = 2 \displaystyle \sqrt{x-3}+ \sqrt{2x-2} = 2 .


The answer is 3.00.

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2 solutions

Chew-Seong Cheong
Jun 16, 2016

x 3 + 2 x 2 = 2 Squaring both sides x 3 + 2 ( x 3 ) ( 2 x 2 ) + 2 x 2 = 4 Rearranging 3 x 9 = 2 ( x 3 ) ( 2 x 2 ) Squaring both sides 9 ( x 3 ) 2 = 4 ( x 3 ) ( 2 x 2 ) Rearranging ( x 3 ) ( 9 x 27 8 x + 8 ) = 0 ( x 3 ) ( x 19 ) = 0 x = { 3 3 3 + 2 ( 3 ) 2 = 2 Accepted 19 19 3 + 2 ( 19 ) 2 = 10 2 Rejected \begin{aligned} \sqrt{x-3}+\sqrt{2x-2} & = 2 \quad \quad \small \color{#3D99F6}{\text{Squaring both sides}} \\ x-3 + 2\sqrt{(x-3)(2x-2)}+2x-2 & = 4 \quad \quad \small \color{#3D99F6}{\text{Rearranging}} \\ 3x - 9 & = -2\sqrt{(x-3)(2x-2)} \quad \quad \small \color{#3D99F6}{\text{Squaring both sides}} \\ 9(x - 3)^2 & = 4(x-3)(2x-2) \quad \quad \small \color{#3D99F6}{\text{Rearranging}} \\ (x-3)(9x - 27 - 8x + 8) & = 0 \\ (x-3)(x - 19) & = 0 \\ \implies x & = \begin{cases} \color{#3D99F6}{3} & \implies \sqrt{\color{#3D99F6}{3}-3}+\sqrt{2(\color{#3D99F6}{3})-2} \color{#3D99F6}{= 2} & \color{#3D99F6}{\text{Accepted}} \\ \color{#D61F06}{19} & \implies \sqrt{\color{#D61F06}{19}-3}+\sqrt{2(\color{#D61F06}{19})-2} = 10 \color{#D61F06}{\ne 2} & \color{#D61F06}{\text{Rejected}} \end{cases} \end{aligned}

The sum of solutions is 3 \boxed{3} .

Too many squaring steps! But an awesome approach! :D

Arkajyoti Banerjee - 4 years, 12 months ago

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Yes, double yours but only twice. But no substitution.

Chew-Seong Cheong - 4 years, 12 months ago

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Sizes don't matter, variation in methods of solving does. :)

Arkajyoti Banerjee - 4 years, 12 months ago

We have, x 3 + 2 x 2 = 2 \displaystyle \sqrt{x-3}+ \sqrt{2x-2} = 2

x 3 + 2 × x 1 = 2 \implies \displaystyle \sqrt{x-3}+ \sqrt{2} \times \sqrt{x-1} = 2

Let u = x 1 u 2 = x 1 u 2 2 = x 3 u = \sqrt{x-1} \implies u^{2} = x-1 \implies u^{2} - 2 = x-3

Hence, our equation becomes:

u 2 2 = 2 2 u \sqrt{u^{2}-2} = 2 - \sqrt{2}u

Squaring both sides, we have:

u 2 2 = 4 4 2 u + 2 u 2 u 2 4 2 u + 6 = 0 u = 2 , 3 2 u^{2}-2=4-4 \sqrt{2}u + 2u^{2} \implies u^{2} -4 \sqrt{2}u + 6 = 0 \implies u= \sqrt{2}, 3 \sqrt{2}

Since u = x 1 u= \sqrt{x-1} , we have x = 3 x=3 and x = 19 x=19 . But substituting these values in the primitive equation, we find that only x = 3 x=3 is a solution but x = 19 x=19 isn't. So we find that the only legitimate solution to the equation is 3 3 .

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