Math 2.0

Mathematical logic and foundations Proposition

A trigonometric inequality

Statement

Induction is often used to prove inequalities. As an example, we prove that ∣sin⁡nx∣≤n∣sin⁡x∣\left|\sin nx\right| \leq n\left|\sin x\right| for any real number xx and natural number nn.

At first glance, it may appear that a more general version, ∣sin⁡nx∣≤n∣sin⁡x∣\left|\sin nx\right| \leq n\left|\sin x\right| for any real numbers n,xn,x, could be proven without induction; but the case n=12, x=πn = \frac{1}{2},\, x=\pi shows it may be false for non-integer values of nn. This suggests we examine the statement specifically for natural values of nn, and induction is the readiest tool.

Further reading

  1. Wikipedia, page 18881, revision 1373903526, Mathematical induction, section Examples > A trigonometric inequality. adopted
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