已知 $\sin \alpha=\frac{3}{5}$, 且 $\alpha$ 为第二象限角.

(1) 求 $\sin 2 \alpha$ 的值;

(2) 求 $\tan \left(\alpha+\frac{\pi}{4}\right)$ 的值.
(1) $\because \sin \alpha=\frac{3}{5}$, 且 $\alpha$ 为第二象限角, $\therefore \cos \alpha=-\sqrt{1-\sin ^{2} \alpha}=-\frac{4}{5}$,

$\therefore \sin 2 \alpha=2 \sin \alpha \cos \alpha=2 \times \frac{3}{5} \times\left(-\frac{4}{5}\right)=-\frac{24}{25}$;

(2) 由（1）知 $\tan \alpha=\frac{\sin \alpha}{\cos \alpha}=-\frac{3}{4}, \therefore \tan \left(\alpha+\frac{\pi}{4}\right)=\frac{\tan \alpha+\tan \frac{\pi}{4}}{1-\tan \alpha \tan \frac{\pi}{4}}=\frac{-\frac{3}{4}+1}{1-\left(-\frac{3}{4}\right)}=\frac{1}{7}$.