14(练习)将二重积分化成二次积分I=「(x,y)ddy 二先对y积分 i=l la f(x, y)dy D b b I=dxb f(x, y)dy b(1--) D f(x, y)dj二 先对 y 积分 y x o a b y x o a b y x o a b D D D   = a x a b I dx f (x, y)dy . . .   = a b x a I dx b f (x, y)dy   −  = a a x b I x f x y y ( ) d ( , )d + = 1 b y a x . 14. (练习)将二重积分化成二次积分 .  = D I f (x, y)dxdy