2018 AMC 12A Problem 9

Problem 9 of 25EasierAlgebra Geometry

Which of the following describes the largest subset of values of

y

within the closed interval

[0, \pi]

for which

\sin(x + y) \le \sin(x) + \sin(y)

for every

x

between

0

and

\pi,

inclusive?

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Solution

For

0 \le x \le \pi

and

0 \le y \le \pi

we have

\sin x \ge 0,

\sin y \ge 0,

\cos x \le 1,

and

\cos y \le 1.

Hence

\begin{aligned} \sin(x+y) &= \sin x \cos y \\ &\quad {}+ \cos x \sin y \\ &\le \sin x + \sin y. \end{aligned}

The inequality therefore holds for every

y

with

0 \le y \le \pi.

Thus, the correct answer is E.

More practice

Concepts: trigonometric identity · inequality

Problem text and solution from the LIVE past-contest archive. See also the AoPS wiki page for community solutions.