flashcrazy f <- expression(x + x^2 + x^4) f <- expression(x^3 + y^5 + (x^6 * y^7)) #对x,y求一阶偏导,这样对不对? D(f, "y") D(f, "y") D(D(f, "x"), "y") #这样为何行不通?R里没反应呢? D(D(f, "y"), "x") D(D(f, "y"), "y")