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Algebra Level 4

If a 3 3 a b 2 = 1818 a^3-3ab^2=1818 and b 3 3 a 2 b = 2222 b^3-3a^2b=2222 find the value of a 2 + b 2 a^2+b^2 .


The answer is 202.

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2 solutions

Culver Kwan
May 18, 2020

( a 2 + b 2 ) 3 = ( a 3 3 a b 2 ) 2 + ( b 3 3 a 2 b ) 2 = 181 8 2 + 222 2 2 \big(a^2+b^2\big)^3=\big(a^3-3ab^2\big)^2+\big(b^3-3a^2b\big)^2=1818^2+2222^2 So a 2 + b 2 = 8242408 3 = 202 a^2+b^2=\sqrt[3]{8242408}=\boxed{202}

Ughhhh, I hit view explanation by mistake!

Mahdi Raza - 1 year ago

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Oh no, you missed a chance of answering this question!

Culver Kwan - 1 year ago
Vilakshan Gupta
May 18, 2020

Slightly motivated but similar solution as @Culver Kwan 's solution:


Note that ( a i b ) 3 = a 3 3 a b 2 + i ( b 3 3 a b 2 ) (a-ib)^3=a^3-3ab^2+i(b^3-3ab^2) and similarly ( a + i b ) 3 = a 3 3 a b 2 i ( b 3 3 a b 2 ) (a+ib)^3=a^3-3ab^2-i(b^3-3ab^2)

Now multiplying both of the above equation yields ( a 2 + b 2 ) 3 = ( a 3 3 a b 2 ) 2 + ( b 3 3 a 2 b ) 2 (a^2+b^2)^3=(a^3-3ab^2)^2+(b^3-3a^2b)^2 .

Hence the answer is ( 181 8 2 + 222 2 2 ) 1 / 3 = 202 (1818^2+2222^2)^{1/3}=\boxed{202} .


Note: i = 1 i=\sqrt{-1} denotes the imaginary unit.

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