1969 Canadian MO Problems/Problem 3

Problem

Let $c$ be the length of the hypotenuse of a right triangle whose two other sides have lengths $a$ and $b$ . Prove that $a+b\le c\sqrt{2}$ . When does the equality hold?

Solution

By the Pythagorean Theorem and the trivial inequality, $2c^2-(a+b)^2=2(a^2+b^2)-(a+b)^2=(a-b)^2\ge 0$ .

Thus $2c^2\ge (a+b)^2.$ Since $a,b,c$ are all positive, taking a square root preserves the inequality and we have our result.

For equality to hold we must have $(a-b)^2 = 0$ . In this case, we have an isosceles right triangle, and equality certainly holds for all such triangles.

1969 Canadian MO (Problems)
Preceded by Problem 2	1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 •	Followed by Problem 4

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1969 Canadian MO Problems/Problem 3

Problem

Solution