在Rt $\triangle A B C$ 中, $\angle C=90^{\circ}, \angle B=4 \angle A$, 求 $\angle B$ 度数.

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解: $\because \angle B=4 \angle A$,

$\therefore \angle A=\frac{1}{4} \angle B$,

$\because \angle C=90^{\circ}$,

$\therefore \angle A+\angle B=90^{\circ}$,

$\therefore \frac{1}{4} \angle B+\angle B=90^{\circ}$,

解得 $\angle B=72^{\circ}$.