國的肌何质( Properties of Plane Areas)國□ 1.组合截面静矩( The first moments of a composite area) S=∑Az;S:=∑4 其中A1-第个简单截面面积 (z;,y—第诠个简单截面的形心坐标 2.组合截面形心( Centroid of a composite area) ∑ ∑Ay ∑A ∑A(Properties of Plane Areas) 其中 Ai —第 i个简单截面面积 1.组合截面静矩(The first moments of a composite area) 2.组合截面形心(Centroid of a composite area) S A zi n i y  i = = 1 = = n i z i i S A y 1 (zi , yi )—第 i个简单截面的形心坐标   = = = n i i n i i i A A z z 1 1   = = = n i i n i i i A A y y 1 1