16解Gm1m2/R^2=m14π^2r1/T^2=m24π^2r2/T^2
R=r1+r2
M=m1+m2
由Gm1m2/R^2=m14π^2r1/T^2得m2=4π^2r1R^2/GT^2
由Gm1m2/R^2=m24π^2r2/T^2得m1=4π^2r2R^2/GT^2
所以M=m1+m2=4π^2（r1+r2）R^2/GT^2=4π^2R^3/GT^2
17解L=gt^2/2
GMm/R^2=mg
M=2LR^2/Gt^2
18解①t1/T1-t1/T2=1
t1=T1T2/(T2-T1)
②t2/T1-t2/T2=1/2
t2=T1T2/2(T2-T1)