S长=长*宽 C长=（长+宽）*2
=ab =（a+b）*2
S正=边长*边长 C正=边长*4
=a*a =4a
S平=底*高
=ah
S三=底*高/2
=ah/2
S梯=（上底+下底）*高/2
=（a+b）*h/2
S圆=圆周率*半径*半径 C圆=2*半径*圆周率
=πr* =2πr