Quadruple Angle Formulas/Sine/Mistake

Source Work

 * Chapter $1$: Complex Numbers
 * Supplementary Problems: $93 \ \text{(a)}$

This mistake can be seen in the 1981 printing of the second edition (1974) as published by Schaum: ISBN 0-070-84382-1

Mistake

 * $\dfrac {\sin \left({4 \theta}\right)} {\sin \theta} = 8 \cos^3 \theta - 4$

The correct expression is:
 * $\dfrac {\sin \left({4 \theta}\right)} {\sin \theta} = 8 \cos^3 \theta - 4 \cos \theta$

as is demonstrated in Quadruple Angle Formulas/Sine/Corollary 1.