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Algebra Level 3

If x + y = 1 x+y=1 and x 2 + y 2 = 2 x^2+y^2=2 , then x 3 + y 3 = a b x^3+y^3=\dfrac{a}{b} for coprime positive integers a a and b b . What is a × b = ? a \times b=\boxed{?}

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The answer is 10.

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2 solutions

Noel Lo
Jun 27, 2017

x + y = 1 x+y=1

( x + y ) 2 = 1 2 (x+y)^2=1^2

x 2 + y 2 + 2 x y = 1 x^2+y^2+2xy=1

2 + 2 x y = 1 2+2xy=1

2 x y = 1 2xy=-1

x y = 1 2 xy=-\frac{1}{2}

Therefore, x 3 + y 3 = ( x + y ) ( x 2 + y 2 x y ) = 1 ( 2 ( 1 2 ) ) = 2 + 1 2 = 2 × 2 + 1 2 = 4 + 1 2 = 5 2 x^3+y^3=(x+y)(x^2+y^2-xy)=1(2-(-\frac{1}{2}))=2+\frac{1}{2}=\frac{2\times2+1}{2}=\frac{4+1}{2}=\frac{5}{2} .

Therefore, a × b = 5 × 2 = 10 a\times b=5\times2=\boxed{10}

same solution

A Former Brilliant Member - 3 years, 11 months ago
Majed Kalaoun
Jul 6, 2017

x + y = 1 x+y=1

( x + y ) 2 = 1 x 2 + y 2 + 2 x y = 1 (x+y)^2=1\Rightarrow x^2+y^2+2xy=1

x 2 + y 2 = 2 x^2+y^2=2

2 + 2 x y = 1 x y = 1 2 2+2xy=1\Rightarrow xy=-\dfrac{1}{2}

( x + y ) 3 = 1 x 3 + y 3 + 3 x 2 y + 3 x y 2 = 1 (x+y)^3=1\Rightarrow x^3+y^3+3x^2y+3xy^2=1

x 3 + y 3 + 3 x y ( x + y ) = 1 x 3 + y 3 + 3 × 1 2 × 1 = 1 x 3 + y 3 = 5 2 x^3+y^3+3xy(x+y)=1\Rightarrow x^3+y^3+3 \times-\dfrac{1}{2}\times1=1\Rightarrow x^3+y^3=\dfrac{5}{2}

a × b = 5 × 2 = 10 a\times b=5\times2=\boxed{10}

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