简单计算一下即可，答案如图所示
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
