Olympiad Maths Prep

Track / Stage 4 / 178 of 340 #438 of 2000

Problem 438

AMC 12 late, AIME early
Algebra Difficulty 4.8 Find the answer

11. Real numbers x,y(1,+)x, y \in(1,+\infty), and xy2xy+1=0x y-2 x-y+1=0, find the minimum value of 32x2+y2\frac{3}{2} x^{2}+y^{2}.

Official solution

y=2x1x1=2+1x132x2+y2=32x2+1(x1)2+4x1+4, y=\frac{2 x-1}{x-1}=2+\frac{1}{x-1} \Rightarrow \frac{3}{2} x^{2}+y^{2}=\frac{3}{2} x^{2}+\frac{1}{(x-1)^{2}}+\frac{4}{x-1}+4,

Let f(x)=32x2+1(x1)2+4x1+4,f(x)=3x2(x1)34(x1)2f(x)=\frac{3}{2} x^{2}+\frac{1}{(x-1)^{2}}+\frac{4}{x-1}+4, f^{\prime}(x)=3 x-\frac{2}{(x-1)^{3}}-\frac{4}{(x-1)^{2}}, since f(x)f^{\prime}(x) is monotonically increasing on (1,+)(1,+\infty), and f(x)=03x=2+4(x1)(x1)33x(x1)3=4x2f^{\prime}(x)=0 \Rightarrow 3 x=\frac{2+4(x-1)}{(x-1)^{3}} \Rightarrow 3 x(x-1)^{3}=4 x-2 (x2)(3x33x2+3x1)=0x=2\Rightarrow(x-2)\left(3 x^{3}-3 x^{2}+3 x-1\right)=0 \Rightarrow x=2, so f(x)min=f(2)=6+1+8=15f(x)_{\min }=f(2)=6+1+8=15.

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.