先求偏导
af/ax=2x/(x^2+y^2+z^2)将点代入=2/9
af/ay=2y/(x^2+y^2+z^2)将点代入=4/9
af/az=2z/(x^2+y^2+z^2)将点代入=-4/9
gradf|M=(2/9)i+(4/9)j+(-4/9)k
先求偏导
af/ax=2x/(x^2+y^2+z^2)将点代入=2/9
af/ay=2y/(x^2+y^2+z^2)将点代入=4/9
af/az=2z/(x^2+y^2+z^2)将点代入=-4/9
gradf|M=(2/9)i+(4/9)j+(-4/9)k