1:9/25*17/26+13/25*9/26
=9/25*17/26+9/25*13/26
=9/25*（17/26+13/26）
=9/25*30/26←上下约分
=27/65
2:5/3*1/12+1/3*1/4
=5/3*1/12+1/12
=1/12*（5/3+1）
=1/12*8/3←上下约分
=2/9
3:7/11*6/8+2/12*6/11
=6/11*7/8+5/12*6/11
=6/11*（7/8+5/12）
=6/11*（21/24+10/24）
=6/11*31/24
=31/44