№1 a) 7/15 + 7/12 №2 Выполни действия 6) 7/9-5/12 в) 8/15-4/21-1/7= а) при с=23/24 выражение 9/12 + c/24= 6) при а=13/21 выражение 6/7 - a/21= №3 Реши уравнение а) У-3/28 = 12/35 6) x + 1/8= 5/8-1/7

а) \[\frac{7}{15} + \frac{7}{12} = \frac{7 \cdot 4}{15 \cdot 4} + \frac{7 \cdot 5}{12 \cdot 5} = \frac{28}{60} + \frac{35}{60} = \frac{28 + 35}{60} = \frac{63}{60} = \frac{21}{20} = 1\frac{1}{20}\]

б) \[\frac{7}{9} - \frac{5}{12} = \frac{7 \cdot 4}{9 \cdot 4} - \frac{5 \cdot 3}{12 \cdot 3} = \frac{28}{36} - \frac{15}{36} = \frac{28 - 15}{36} = \frac{13}{36}\]

в) \[\frac{8}{15} - \frac{4}{21} - \frac{1}{7} = \frac{8 \cdot 7}{15 \cdot 7} - \frac{4 \cdot 5}{21 \cdot 5} - \frac{1 \cdot 15}{7 \cdot 15} = \frac{56}{105} - \frac{20}{105} - \frac{15}{105} = \frac{56 - 20 - 15}{105} = \frac{21}{105} = \frac{1}{5}\]

а) при с=\(\frac{23}{24}\) выражение \(\frac{9}{12} + \frac{c}{24}=\) \[\frac{9}{12} + \frac{c}{24} = \frac{9}{12} + \frac{23}{24} : \frac{9 \cdot 2}{12 \cdot 2} + \frac{23}{24} = \frac{18}{24} + \frac{23}{24} = \frac{18 + 23}{24} = \frac{41}{24} = 1\frac{17}{24}\]

б) при а=\(\frac{13}{21}\) выражение \(\frac{6}{7} - \frac{a}{21}=\) \[\frac{6}{7} - \frac{a}{21} = \frac{6}{7} - \frac{13}{21} = \frac{6 \cdot 3}{7 \cdot 3} - \frac{13}{21} = \frac{18}{21} - \frac{13}{21} = \frac{18 - 13}{21} = \frac{5}{21}\]

а) У - \(\frac{3}{28} = \frac{12}{35}\) \[У = \frac{12}{35} + \frac{3}{28} = \frac{12 \cdot 4}{35 \cdot 4} + \frac{3 \cdot 5}{28 \cdot 5} = \frac{48}{140} + \frac{15}{140} = \frac{48 + 15}{140} = \frac{63}{140} = \frac{9}{20}\]

б) x + \(\frac{1}{8} = \frac{5}{8} - \frac{1}{7}\) \[x = \frac{5}{8} - \frac{1}{7} - \frac{1}{8} = \frac{5 \cdot 7}{8 \cdot 7} - \frac{1 \cdot 8}{7 \cdot 8} - \frac{1 \cdot 7}{8 \cdot 7} = \frac{35}{56} - \frac{8}{56} - \frac{7}{56} = \frac{35 - 8 - 7}{56} = \frac{20}{56} = \frac{5}{14}\]