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Algebra Difficulty 5.0 AIME Find the answer

Over all real numbers xx and yy, find the minimum possible value of (xy)2+(x+7)2+(2y+7)2 (x y)^{2}+(x+7)^{2}+(2 y+7)^{2}

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

Solution 1: Rewrite the given expression as (x2+4)(1+y2)+14(x+2y)+94\left(x^{2}+4\right)\left(1+y^{2}\right)+14(x+2 y)+94. By Cauchy-Schwartz, this is at least (x+2y)2+14(x+2y)+94=(x+2y+7)2+45(x+2 y)^{2}+14(x+2 y)+94=(x+2 y+7)^{2}+45. The minimum is 45 , attained when xy=2,x+2y=7x y=2, x+2 y=-7. Solution 2: Let z=2y,s=x+z,p=xzz=2 y, s=x+z, p=x z. We seek to minimize (xz2)2+(x+7)2+(z+7)2=p24+(x2+z2)+14(x+z)+98=p24+s22p+14s+98=(p22)2+(s+7)2+4545\begin{aligned} \left(\frac{x z}{2}\right)^{2}+(x+7)^{2}+(z+7)^{2} & =\frac{p^{2}}{4}+\left(x^{2}+z^{2}\right)+14(x+z)+98 \\ & =\frac{p^{2}}{4}+s^{2}-2 p+14 s+98 \\ & =\left(\frac{p}{2}-2\right)^{2}+(s+7)^{2}+45 \\ & \geq 45 \end{aligned} Equality holds when s=7,p=4s=-7, p=4. Since s24ps^{2} \geq 4 p, this system has a real solution for xx and zz.

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