计算（a)图截面形心坐标Yc:
大矩形：S1 = (100)(110) = 11000, Yc1 = -55
小矩形：S2 = -(80)(100) = -8000, Yc2 = -60
Yc =(Yc1.S1 +Yc2.S2)/(S1+S2)
=[(-55)(11000) +(-60)(-8000)]/(11000 -8000)
≈ - 42
.
计算（b）图截面惯性矩Iz:
题目未指明尺寸单位，暂且假定为mm
大矩形： Iz1 = (1/12)(10cm)(14cm)^3 ≈ 2287cm^4
左右2个小矩形：Iz2 = (1/12)(4cm)(10cm)^3 ≈ 333cm^4
Iz = Iz1 -2Iz2 = 2287cm^4 - 2(333cm^4) = 1621cm^4
