math
posted by ayan .
The point (x,y) lies on both conics x2+xy+x=81 and y2+xy+y=51. Given that x+y is positive, determine the value of x+y.

x^2 + xy + x = 81
xy + y^2 + y = 51
add them to get
x^2 + 2xy + y^2 + x + y = 132
(x+y)^2 + (x+y)  132 = 0
((x+y)+13)((x+y)12) = 0
so, x+y = 12
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