已知∠AOB=50°,从O点再引一条射线OC,使∠BOC=20°,求∠AOC的度数.
–Which hair band do you like best, madam?–I can't decide now. I really don't know _________.
一个物体只受F1和F2两个力的作用,这两个力的三要素完全相同,则这个物体所受到的合力是( )