Dizzy algebra

Algebra Level 2

What is the value of the following expression? ( x y ) 3 + ( y z ) 3 + ( z x ) 3 ( x y ) ( y z ) ( z x ) \frac { { (x-y) }^{ 3 }+{ (y-z) }^{ 3 }+{ (z-x) }^{ 3 } }{ (x-y)(y-z)(z-x) }


The answer is 3.

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5 solutions

Milind Prabhu
Mar 14, 2014

Let ( x y ) = a ( y z ) = b ( z x ) = c (x-y)=a\quad (y-z)=b\quad (z-x)=c

the question is of the form a 3 + b 3 + c 3 a b c \frac { { a }^{ 3 }+{ b }^{ 3 }+{ c }^{ 3 } }{ abc }

( x y ) + ( y z ) + ( z x ) = 0 (x-y)+(y-z)+(z-x)=0

so a + b + c = 0 a+b+c=0

( a + b ) = c (a+b)=-c

( a + b ) 3 = ( c ) 3 { (a+b) }^{ 3 }={ (-c) }^{ 3 }

a 3 + b 3 + 3 a b ( a + b ) = c 3 { a }^{ 3 }+{ b }^{ 3 }+3ab(a+b)={ -c }^{ 3 }

because ( a + b ) = c (a+b)=-c we get

a 3 + b 3 + 3 a b ( c ) = c 3 { a }^{ 3 }+{ b }^{ 3 }+3ab(-c)={ -c }^{ 3 }

a 3 + b 3 + c 3 = 3 a b c { a }^{ 3 }+{ b }^{ 3 }+{ c }^{ 3 }=3abc

a 3 + b 3 + c 3 a b c = 3 \frac { { a }^{ 3 }+{ b }^{ 3 }+{ c }^{ 3 } }{ abc } =3

So the answer is 3 3

a very good solution!

Mayank Holmes - 7 years ago

this was what I did

Max B - 7 years ago
Ashish Jaisawal
Mar 20, 2014

just put x=1,y=2 and z=3 and solve you get the answer

lol. Indians.

Shuchit Khurana - 7 years, 1 month ago
Rahul Gautam
Apr 4, 2014

Shortcut :- Just plug in the random values for the variables (x,y,z) => (1,2,3) => {(1-2)^3+(2-3)^3+(3-1)^3}/(1-2)(2-3)(3-1) => (-1-1+8)/(-1)(-1)(2) => 3

Let ( x y ) = a ( y z ) = b ( z x ) = c (x-y)=a \quad (y-z)=b \quad (z-x)=c .

Then, ( x y ) 3 + ( y z ) 3 + ( z x ) 3 ( x y ) ( y z ) ( z x ) = a 3 + b 3 + c 3 a b c \frac{(x-y)^{3}+(y-z)^{3}+(z-x)^{3}}{(x-y)(y-z)(z-x)}=\frac{a^{3}+b^{3}+c^{3}}{abc}

a 3 + b 3 + c 3 a b c = a 3 + b 3 + c 3 3 a b c a b c + 3 \frac{a^{3}+b^{3}+c^{3}}{abc}=\frac{a^{3}+b^{3}+c^{3}-3abc}{abc}+3

a 3 + b 3 + c 3 a b c = ( a + b + c ) ( a 2 + b 2 + c 2 a b b c a c ) a b c + 3 \frac{a^{3}+b^{3}+c^{3}}{abc}=\frac{(a+b+c)(a^{2}+b^{2}+c^{2}-ab-bc-ac)}{abc}+3

Because a + b + c = ( x y ) + ( y z ) + ( z x ) = 0 \quad a+b+c=(x-y)+(y-z)+(z-x)=0

Thus, the value of ( x y ) 3 + ( y z ) 3 + ( z x ) 3 ( x y ) ( y z ) ( z x ) \frac{(x-y)^{3}+(y-z)^{3}+(z-x)^{3}}{(x-y)(y-z)(z-x)} is 3 \boxed{3}

Moshiur Mission
Mar 30, 2014

putting a = y-z, b = z-x, c = x-y we get a+b+c=0 we get equation is (a3+b3+c3)/abc = (a3+b3+c3-3abc+3abc)/abc =((a+b+c)(a2+b2+c2-ab-bc-ca) +3abc)/abc = 3abc/abc=3

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