Maths Olympiad Prep

Library / /17 of 28

Algebra Difficulty 4.8 AIME Prove it Philippines

Problem:
Find the minimum value of x2+4y22xx^{2}+4 y^{2}-2 x, where xx and yy are real numbers that satisfy 2x+8y=32 x+8 y=3.

Solution

Solution:
By the Cauchy-Schwarz Inequality,
[2(x1)+4(2y)]2(22+42)[(x1)2+4y2] [2(x-1)+4(2 y)]^{2} \leq (2^{2}+4^{2})\left[(x-1)^{2}+4 y^{2}\right]
Now, with 2(x1)+4(2y)=2x+8y2=32=12(x-1)+4(2 y)=2 x+8 y-2=3-2=1, we obtain
x2+4y22x=(x1)2+4y21[2(x1)+4(2y)]222+421=1201=1920 \begin{aligned} x^{2}+4 y^{2}-2 x & =(x-1)^{2}+4 y^{2}-1 \\ & \geq \frac{[2(x-1)+4(2 y)]^{2}}{2^{2}+4^{2}}-1 \\ & =\frac{1}{20}-1 \\ & =-\frac{19}{20} \end{aligned}
The minimum 1920-\frac{19}{20} is indeed obtained with x=1110x=\frac{11}{10} and y=110y=\frac{1}{10}.

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.