若直线l1:ax+(3﹣a)y+1=0,l2:2x﹣y=0,若l1⊥l2,则实数a的值为____
一个圆锥的母线长是底面半径的2倍,则侧面展开图扇形的圆心角是( )
- Where are you from?
- ___________()