第13章 13.9

$\sin x + \cos x = \frac{6}{5}$

$(\sin x + \cos x)^2 = \frac{36}{25} \qquad \sin^2 x + 2\sin x \cos x + \cos^2 x = \frac{36}{25} \qquad 2\sin x \cos x = \frac{36}{25} - 1 = \frac{11}{25}$

$\sin 2x = 2\sin x \cdot \cos x = \frac{11}{25}$