A:3﹣1=﹣3
B:a2•a3=a6
C:(x+1)2=x2+1
D:32−2=22
He is a strict teacher, but all the students like him. I don't know why.
如图是健身用的“跑步机”示意图。质量为m的健身者踩在与水平面成α角的静止皮带上,用力向后蹬皮带,可使皮带以速度v匀速向后运动。若皮带在运动的过程中受到脚的摩擦力为f,则在运动的过程中,下列说法中正确的是( )