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Algebra Difficulty 2.8 Junior Find the answer

Suppose that xx and yy satisfy xyx+y=9\frac{x-y}{x+y}=9 and xyx+y=60\frac{xy}{x+y}=-60. What is the value of (x+y)+(xy)+xy(x+y)+(x-y)+xy?

A number or a short expression. Spacing and $ signs are ignored.

Solution

The first equation xyx+y=9\frac{x-y}{x+y}=9 gives xy=9x+9yx-y=9x+9y and so 8x=10y-8x=10y or 4x=5y-4x=5y. The second equation xyx+y=60\frac{xy}{x+y}=-60 gives xy=60x60yxy=-60x-60y. Multiplying this equation by 5 gives 5xy=300x300y5xy=-300x-300y or x(5y)=300x60(5y)x(5y)=-300x-60(5y). Since 5y=4x5y=-4x, then x(4x)=300x60(4x)x(-4x)=-300x-60(-4x) or 4x2=60x-4x^{2}=-60x. Rearranging, we obtain 4x260x=04x^{2}-60x=0 or 4x(x15)=04x(x-15)=0. Therefore, x=0x=0 or x=15x=15. Since y=45xy=-\frac{4}{5}x, then y=0y=0 or y=12y=-12. From the first equation, it cannot be the case that x=y=0x=y=0. We can check that the pair (x,y)=(15,12)(x, y)=(15,-12) satisfies both equations. Therefore, (x+y)+(xy)+xy=3+27+(180)=150(x+y)+(x-y)+xy=3+27+(-180)=-150.

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