4.设x=x2yf(xy,g(x,y),y=p(x),求; 5.已知(axy3-yosx)dx+(1+ basin.x+3x2y2)dy为某一函数f(x,y)的全微分, 求a和b的值; 6.设函数z=f(x,y),若2=2,且f(x,0)=1,,(x,0)=x,求f(x,y);