An algebra problem by ismet cosic

Algebra Level 3

{ x z = y 8 3 y z = x 2 3 z = x 4 + y 4 \large \begin{cases} x^{z}=y^{\frac{8}{3}} \\ y^{z}=x^{\frac{2}{3}} \\ z=\sqrt[4]{x}+\sqrt[4]{y} \end{cases}

There are two triplets ( x , y , z ) (x,y,z) of solutions to this system of equations, one of which is ( 1 , 1 , 2 ) (1,1,2) . If the other set is ( a , b , c ) (a,b,c) , what is the value of a b × c \dfrac{\sqrt{a}}{b} \times c rounded to three decimal places?


The answer is 1.333.

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1 solution

Ismet Cosic
Jan 5, 2017

Considering the third equation, x , y , z > 0 x,y,z > 0 . ( 0 , 0 , 0 ) (0,0,0) isn't a solution because 0 0 0^{0} is not defined.

y z = x 2 3 y^{z} =x^{\frac{2}{3}}

y z × z = x 2 3 × z = ( x z ) 2 3 = ( y 8 3 ) 2 3 = y 16 9 y^{z\times z}=x^{\frac{2}{3} \times z} = (x^{z})^{\frac{2}{3}}= (y^{\frac{8}{3}})^{\frac{2}{3}} = y^{\frac{16}{9}}

y z 2 = y 16 9 y^{z^{2}} = y^{\frac{16}{9}} This means that either y = 1 y=1 or z 2 = 16 9 z^{2}=\frac{16}{9} . For y = 1 y=1 we get the first given set of solutions ( 1 , 1 , 2 ) (1,1,2) .

For z 2 = 16 9 z^{2}=\frac{16}{9} we get z = 4 3 z=\frac{4}{3} , meaning c = 4 3 c=\frac{4}{3} .

From the first equation, we get x 4 3 = y 8 3 x^{\frac{4}{3}} = y^{\frac{8}{3}} , or x = y 2 x = y^{2} .

From this point, we could solve for x and y, but that's unnecessary because we know that a = b 2 a = b^{2} .

a b × c = b 2 b × 4 3 = b b × 4 3 = 1 × 4 3 = 4 3 = 1.333 \frac{\sqrt{a}}{b} \times c=\frac{\sqrt{b^{2}}}{b} \times \frac{4}{3}= \frac{b}{b} \times \frac{4}{3}= 1 \times \frac{4}{3}= \frac{4}{3} =\boxed{1.333}

why discard the negative y? for example (1, -1, 2) is also a solution

David Qian - 4 years, 5 months ago

For the third equation, is one of the terms supposed to be y y ? Otherwise, can we make it 2 x 4 2 \sqrt[4]{x} directly?

Calvin Lin Staff - 4 years, 5 months ago

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