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Algebra Level 2

If a 2 + a b + b 2 = 19 a^{2} + ab + b^{2} = 19 a 4 + a 2 b 2 + b 4 = 133 a^{4}+a^{2}b^{2}+b^{4} = 133

then what is the value of a b \boxed{ab} ? ?

7 6 9 8

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3 solutions

Jose Dehilario
Jul 25, 2015

a 4 + a 2 b 2 + b 4 a 2 + a b + b 2 = a 2 a b + b 2 \frac { { a }^{ 4 }+{ a }^{ 2 }{ b }^{ 2 }+{ b }^{ 4 } }{ { a }^{ 2 }+ab+{ b }^{ 2 } } ={ a }^{ 2 }-ab+{ b }^{ 2 } 133 19 = a 2 a b + b 2 \frac { 133 }{ 19 } ={ a }^{ 2 }-ab+{ b }^{ 2 } a 2 a b + b 2 = 7 { a }^{ 2 }-ab+{ b }^{ 2 }=7 a 2 + a b + b 2 ( a 2 a b + b 2 ) = 19 7 { a }^{ 2 }+ab+{ b }^{ 2 }-({ a }^{ 2 }-ab+{ b }^{ 2 })=19-7 2 a b = 12 2ab=12 a b = 6 ab=6

Praduman Singh
Jul 25, 2015

squaring the first eqn a 4 + a 2 b 2 + b 4 + 2 a b ( a 2 + b 2 + a b ) = 361 a^{4} + a^{2}b^{2} + b^{4} + 2ab(a^{2} + b^{2} +ab) = 361 133 + 2 a b 19 = 361 133 + 2ab*19 = 361 hence a b = 6 ab = 6

Ikkyu San
Jul 25, 2015

a 4 + a 2 b 2 + b 4 = ( a 4 + 2 a 2 b 2 + b 4 ) a 2 b 2 133 = ( a 2 + b 2 ) 2 ( a b ) 2 133 = ( a 2 + b 2 + a b ) ( a 2 + b 2 a b ) 133 = 19 [ ( a 2 + a b + b 2 ) 2 a b ] 7 = 19 2 a b 12 = 2 a b a b = 6 \begin{aligned}a^4+a^2b^2+b^4=&\ (a^4+2a^2b^2+b^4)-a^2b^2\\133=&\ (a^2+b^2)^2-(ab)^2\\133=&\ (a^2+b^2+ab)(a^2+b^2-ab)\\133=&\ 19[(a^2+ab+b^2)-2ab]\\7=&\ 19-2ab\\-12=&\ -2ab\\ab=&\ \boxed6\end{aligned}

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