3.计算下面各题.1/3乘以3/5+1 5/7-5/9乘以5/7 1-5/7乘以21/25 1/2+4/5乘以4/5 1/6乘以（5-2/3） =1/5+1 =（1-5/9）乘以5/7 =1-3/5 =1/2+1 =1/6乘以5-1/6乘以2/3=1又1/5 =4/9乘以5/7 =2/5 =1又1/2 =5/6-1/9=20/63 =13...