(2) F(x,y) 20 2(16+2)(25+1chhv 20 2J-∞16+ ∞025+1 又ar+ 201 x)1 arte t 42)5 52 x 1 arct C、D 42 52       +      = +        +      =  + +  + = + + =     − − − − 2 1 5 1 2 1 4 1 5 5 2 1 4 4 2 20 1 25 1 16 20 1 (16 )(25 ) 20 (2). ( , ) 2 2 2 2 2 2 2 y arctg x arctg y arctg x arctg dv v du u dudv u v F x y x y y x       