Back to 1957!

Algebra Level pending

Given that x x and y y are real numbers such that x 3 + y 3 = 1957 x^{3}+y^{3}=1957 and ( x + y ) ( x + 1 ) ( y + 1 ) = 2014 (x+y)(x+1)(y+1)=2014 , find the value of x + y x+y .


The answer is 19.

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1 solution

Victor Loh
Feb 22, 2016

Let x + y = a x+y=a and x y = b xy=b . Then x 3 + y 3 = 1957 ( x + y ) 3 3 x y ( x + y ) = a 3 3 a b = 1957 ( 1 ) . x^{3}+y^{3}=1957\\ \implies (x+y)^{3}-3xy(x+y)=a^{3}-3ab=1957 \cdots (1). ( x + y ) ( x + 1 ) ( y + 1 ) = 2014 ( x + y ) ( x + y + x y + 1 ) = a ( a + b + 1 ) = a 2 + a b + a = 2014 ( 2 ) . (x+y)(x+1)(y+1)=2014\\ \implies (x+y)(x+y+xy+1)=a(a+b+1)=a^{2}+ab+a=2014 \cdots (2). From ( 1 ) (1) , a b = a 3 1957 3 ( 3 ) . ab=\frac{a^{3}-1957}{3}\cdots(3). Substituting ( 3 ) (3) into ( 2 ) (2) , a 2 + a 3 1957 3 + a = 2014 3 a 2 + a 3 1957 + 3 a = 6042 a 3 + 3 a 2 + 3 a 7999 = 0 a 3 + 3 a 2 + 3 a + 1 = 8000 ( a + 1 ) 3 = 2 0 3 a = x + y = 20 1 = 19 . a^{2}+\frac{a^{3}-1957}{3}+a=2014\\ 3a^{2}+a^{3}-1957+3a=6042\\ a^{3}+3a^{2}+3a-7999=0\\ a^{3}+3a^{2}+3a+1=8000\\ (a+1)^{3}=20^{3} \implies a=x+y=20-1=\boxed{19}.

Same Way!!! :D

Raffy Chan - 5 years, 2 months ago

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