Problem: Suppose x and y are positive real numbers such that x+y1=y+x2=3. Compute the maximum possible value of xy.
Solutions — 2
Solution 1
Solution: Rewrite the equations as xy+1=3y and xy+2=3x. Let xy=C, so x=3C+2 and y=3C+1. Then (3C+2)(3C+1)=C⟹C2−6C+2=0. The larger of its two roots is 3+7.
Solution 2
Solution: Multiply the two equations to get xy+3+xy2=9, so letting C=xy gives C2−6C+2=0, which has larger root C=3+7.
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