1)
(4в-5)/10 - (3-5в)/10 = (4в-5 - 3+5в)/10 = (9в-8)/10,
2)
(х-2)/(7-х) - (2+х)/(7-х) = (х-2 - 2-х)/(7-х) = -4/(7-х),
3)
(3а+4)/а - (3а+4в)/ав = в*(3а+4)/ав - (3а+4в)/ав = (3ав+4в - 3а-4в)/ав =
= (3ав-3а)/ав = 3а*(в-1)/ав = 3(в-1)/в,
4)
м/(5-м) + (3-м)/(2м-10) = м/(5-м) + (3-м)/(м-5)*2 =
= -м/(м-5) + (3-м)/(м-5)*2 = -2м/(м-5)*2 + (3-м)/(м-5)*2 =
= (-2м + 3-м)/(м-5)*2 = (-3м+3)/(м-5)*2 = -3(м-1)/2(м-5)