梯形面积1/2(m-n+3n-m)*h1=nh1剩下就是nh1-m
[(m-n+3n+m)*h1]/2-m
(m-n+3n+m)×h÷2=mh+nh
S梯=(上底+下底)*高/2 =(M-N+3N+M)*H1/2 S剩=S梯-S长 ==(M-N+3N+M)*H1/2-M