Chapter 7 - Analytic Trigonometry - 7.6 Double-angle and Half-angle Formulas - 7.6 Assess Your Understanding - Page 497: 2

$1-\cos(\theta)$

Recall the half-angle Identity for sine: $\sin {(\frac{\theta}{2})}=\pm\sqrt{\frac{1-\cos{\theta}}{2}}$ Squaring both sides of the identity above gives $\sin^2 {(\frac{\theta}{2})}=\frac{1-\cos{\theta}}{2}$ Hence, the missing expression in the given statement is $1-\cos{(\theta)}.$