F-(M+m）g=（M+m)a ····················☆

题目

F-(M+m）g=（M+m)a ····················☆
2f-Mg=Ma·····································★
联立上面两个星星,怎么得出!
F= 〔2（m+M）〕/M
F-(M+m）g=（M+m)a ····················☆
2f-Mg=Ma·····································★
联立上面两个星星，怎么得出！
F= 〔2f（m+M）〕/M

答案

(1)得F=(M+m)(g+a)
(2)得2f=M(g+a) (g+a)=2f/M
代入得
F=2f(M+m)/M