>>721
(1)
(-GMm/b)-(-GMm/a)
=GMm{(1/a)-(1/b)}
=GMm(b-a)/ab

(2)エネルギー保存則
(mvv/2)+(-GMm/b) = (muu/2)+(-GMm/a)
(mvv/2) = (muu/2) + GMm(a-b)/ab
vv = uu + 2GM(a-b)/ab
v= √{uu + 2GM(a-b)/ab}