5/21(7-3x)-4/35(5x+7)=12/15-2/7

5/21 * (7 - 3х) - 4/35 * (5х + 7) = 12/15 - 2/75/3 - 5/7х - 4/7х - 4/5 = 28/35 - 10/3525/15 - 12/15 - 9/7х = 18/3513/15 - 9/7х = 18/359/7х = 13/15 - 18/359/7х = 91/105 - 54/1059/7х = 37/105х = 37/105 : 9/7 = 37/105 * 7/9 = (37*1)/(15*9) х = 37/135Пояснения:5/21 * 7 = 35/21 = 5/3 - сократили на 75/21 * 3 = 15/21 = 5/7 -сократили на 34/35 * 5 = 20/35 = 4/7 - сократили на 54/35 * 7 = 28/35 = 4/5 - сократили на 712/15 = 4/5 = 28/35 - сократили на 3 и умножили на 7/72/7 = 10/35 - доп.множ. 55/3 = 25/15 - доп.множ. 54/5 = 12/15 - доп.множ. 313/15 = 91/105 - доп.множ. 718/35 = 54/105 - доп.множ. 3