Problem: Find the minimum value of x2+4y2−2x, where x and y are real numbers that satisfy 2x+8y=3.
Solution
Solution: By the Cauchy-Schwarz Inequality, [2(x−1)+4(2y)]2≤(22+42)[(x−1)2+4y2] Now, with 2(x−1)+4(2y)=2x+8y−2=3−2=1, we obtain x2+4y2−2x=(x−1)2+4y2−1≥22+42[2(x−1)+4(2y)]2−1=201−1=−2019 The minimum −2019 is indeed obtained with x=1011 and y=101.
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Source: MathNet,
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