Is it possible?

Algebra Level 4

Find the sum of all solutions of the following equation: 1 ­ 4 x + 1 ­ 7 x 9 = 1 ­ 5 x 3 + 1 ­ 6 x 6 . \frac { 1 }{ ­\sqrt{ 4x } } +\frac { 1 }{ ­\sqrt{ 7x-9 } } =\frac { 1 }{ ­\sqrt{ 5x-3 } } +\frac { 1 }{ ­\sqrt { 6x-6}} .


The answer is 3.

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2 solutions

Rindell Mabunga
Oct 10, 2014

Let A = 4 x A = \sqrt{4x} , B = 7 x 9 B = \sqrt{7x - 9} , C = 5 x 3 C = \sqrt{5x - 3} , D = 6 x 6 D = \sqrt{6x -6}

The equation will become:

1 A + 1 B = 1 C + 1 D \frac1A + \frac1B = \frac1C + \frac1D

Simplifying this will be hard but upon inspection in the equation, I noticed that

A 2 + B 2 = C 2 + D 2 A^2 + B^2 = C^2 + D^2

Further simplification will result to:

( ( A 2 + B 2 ) ( C D ) + ( A 2 + B 2 ) ( A B ) 2 A B C D ) ( C D A B ) = 0 ((A^2 + B^2)(CD) + (A^2 + B^2)(AB) - 2ABCD)(CD - AB) = 0

The first factor will result to imaginary roots but the second will have real roots.

C D A B = 0 CD - AB = 0

C D = A B CD = AB

5 x 3 × 6 x 6 = 4 x × 7 x 9 \sqrt{5x - 3} \times \sqrt{6x -6} = \sqrt{4x} \times \sqrt{7x - 9}

4 x ( 7 x 9 ) = ( 5 x 3 ) ( 6 x 6 ) 4x(7x - 9) = (5x -3)(6x -6)

28 x 2 36 x = 30 x 2 48 x + 18 28x^2 - 36x = 30x^2 - 48x + 18

0 = 2 x 2 12 x + 18 0 = 2x^2 - 12x + 18

( x 3 ) 2 = 0 (x - 3)^2 = 0

x = 3 x = \boxed{3}

Nice. The guiding principle is to look at the graph of

1 x + 1 k x 2 . \frac{ 1}{ x} + \frac{ 1 } { \sqrt{ k - x^ 2} }.

This tells us that if A 2 + B 2 = C 2 + D 2 ( = k ) A^2 + B^2 = C^2 + D^2 (=k) and 1 A + 1 B = 1 C + 1 D \frac{1}{A} + \frac{1}{B} = \frac{1}{C} + \frac{1}{D} , then we must have { A , B } = { C , D } \{ A, B \} = \{ C, D \} .

Calvin Lin Staff - 6 years, 8 months ago

shortcut sorry

Rindell Mabunga - 6 years, 8 months ago

I would like to see how you simplified it in your first step (the one with squares), please. Mind boggling me for a while ;(

Mohammad Al Ali - 6 years, 8 months ago
Trevor Arashiro
Oct 8, 2014

This solution uses only intuition.

By some dumb guess/luck/genius (not so much the third), I set 6 x 6 = 4 x x = 3 6x-6=4x\Rightarrow x=3

Checking this, I found that the expression became

1 12 + 1 12 = 1 12 + 1 12 \frac{1}{12}+\frac{1}{12}=\frac{1}{12}+\frac{1}{12}

However there is some logic behind this. Since there are all irrational denominators, one on the LHS will MOST LIKELY have to cancel out directly with another on the RHS. Thus setting two equal to each other, we find the answer.

SOME ONE PLEASE VARIFY THIS: The proof that there is only one answer is because the x coefficients add up to the same on both sides.

thats fake

Mehul Chaturvedi - 6 years, 8 months ago

I have a solution wait guys!

Rindell Mabunga - 6 years, 8 months ago

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