(1)s-2t/3s×6s²/s+2t=(s-2t)*2s/(s+2t)
(2)u-2v/u+2v－2/u²-4v²=u-2v/u+2v －2/[(u-2v)(u+2v)]=[(u-2v)²-2]/[(u-2v)(u+2v)]
(3)2a/a+1＋2/a+1=(2a+2)/(a+1)=2(a+1)/(a+1)=2