а) 7/15 + 7/12 = (7/15)(4/4) + (7/12)(5/5) = 28/60 + 35/60 = 63/60 = 21/20
б) 7/9 - 5/12 = (7/9)(4/4) - (5/12)(3/3) = 28/36 - 15/36 = 13/36
в) 5/21 + 5/28 = (5/21)(4/4) + (5/28)(3/3) = 20/84 + 15/84 = 35/84
г) 11/42 - 5/28 = (11/42)(2/2) - (5/28)(3/3) = 22/84 - 15/84 = 7/84
а) 7/15 + 7/12 = (7/15)(4/4) + (7/12)(5/5) = 28/60 + 35/60 = 63/60 = 21/20
б) 7/9 - 5/12 = (7/9)(4/4) - (5/12)(3/3) = 28/36 - 15/36 = 13/36
в) 5/21 + 5/28 = (5/21)(4/4) + (5/28)(3/3) = 20/84 + 15/84 = 35/84
г) 11/42 - 5/28 = (11/42)(2/2) - (5/28)(3/3) = 22/84 - 15/84 = 7/84