13 圖示平面應力元素,$\sigma_{x} = 25 \mathrm{MPa}$、$\sigma_{y} = -15 \mathrm{MPa}$、$\tau_{xy} = -10 \mathrm{MPa}$,則平面最大剪應力多大?
(A) $27.4 \mathrm{MPa}$(B) $17.4 \mathrm{MPa}$(C) $22.4 \mathrm{MPa}$(D) $20 \mathrm{MPa}$