分数简便运算.要求写出过程.

1.1997*137/1998
=(1-1/1998)*137
=137-137/1998
=136又137/1861

2.15+(1/13)/14
=14/14+(14/13)/14
=14/13

3.3/2+7/6+13/12+21/20+31/30+43/42
=1+1/2+1+1/6+1+1/12+1+1/20+1+1/30+1+/42
=6+1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7
=7-1/7
=48/7
=6又6/7

4..（1-1/4）*（1-1/9）*（1-1/16）*（1-1/25）*（1-1/36）*（1-1/49）
==(1-1/2)(1+1/2)(1-1/3)(1+1/3)`````(1+1/7)(1-1/7)
=3/4*8/9*15/16*24/25*35/36*48/49
约=0.5714

5.令x=1/3+1/5+1/7,则原式=(1+x)*(x+1/9)-(1+x+1/9)*x
=1/9