例6 2-8x+25 解 x2-8x+25 x-4)2+9 x-4 32/x-4 +1 +1 3 3 =arctan +c 3例6 求 . 8 25 1 2 dx x x  − + 解 dx x x  − 8 + 25 1 2 dx x  − + = ( 4) 9 1 2 dx x   +      − = 1 3 4 1 3 1 2 2       −  +      − =  3 4 1 3 4 1 3 1 2 x d x . 3 4 arctan 3 1 C x + − =