求图形对y,z轴的静矩 及-- a+ =bh(a+）=A 81=t bh 2 10 2 S.ydd =0 2 h 55 z y  = A z S ydA ZC  + = a h a ybdy a h a by + = 2 2 ) 2 ( h = bh a + = AyC b h a dy y z dz  = A y S zdA  = b zhdz 0 b hz 0 2 2 = 2 b = bh = AzC  = A z S ydA c  − = 2 2 h h ybdy 2 2 2 2 h h by − = = 0 求图形对y、z 轴的静矩