13、 I = ( x + y dxdy , D : x 2 + y 2 1 D 解: y D1 I = 4 ( x + y dxdy D1 0 x = 4 2 0 8 d r (cos + sin rdr = 0 3 1 14、设 f (u 为可微函数, f (0 = 0,求极限 1 lim 3 f t 0 + t x 2 + y 2 t 2 ( x 2 + y 2 d 高等数学Copyright2012 by Samw. All rights reserved. 2t f (t = lim 解: 原式 = tlim 3 + 2 0 t 0 + t 3t 2 f (t f (0 2 = lim = f (0 + t 3 t 0 3 0 16 2 r f (r dr t