單項(xiàng)選擇題

如圖所示單元體中,ab斜面上的正應(yīng)力σa應(yīng)為()

A.(50-20)/2+[(50+20)/2]cos(-60°)-30sin(-60°)
B.(50+20)/2+[(50-20)/2]sin(60°)-30cos(60°)
C.(50+20)/2+[(50-20)/2]cos(-60°)+30sin(-60°)
D.(50-20)/2+[(50+20)/2]cos(-60°)+30sin(-60°)