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а)3/49+5/7+4/49 =
б)1/12+1/18+1/12 +

3/49+5/7+4/49=3/49+35/49+4/49=42/49=6/7; 

1) 3/49+4/49=7/49=1/7

2) 5/7+1/7=6/7

1/12+1/18+1/12=3/36+2/36+3/36=8/36=2/9

1) 1/12+1/12=2/12=1/6

2) 1/6+1/18=3/18+1/18=4/18=2/9