959. Вычислите значение выражения: 1) -\frac{1}{4}+\frac{3}{5}; 2) \frac{9}{11}+(-\frac{2}{5}); 3) \frac{20}{21}+\frac{3}{7}; 4) -4\frac{5}{9}+(-\frac{7}{6}); 5) 7\frac{5}{12}+(-\frac{3}{24}); 6) -6\frac{11}{12}+(-\frac{8}{18}); 7) -13\frac{3}{16}+7\frac{3}{16}; 8) -2\frac{3}{8}+(-1\frac{5}{9}); 9) -3\frac{1}{12}+\frac{1}{6}; 10) 3\frac{6}{7}+(-6\frac{4}{9}); 11) 9\frac{1}{6}+(-\frac{5}{4}); 12) 4\frac{3}{7}+(-\frac{8}{14}).

Решим примеры:

1. $$-\frac{1}{4}+\frac{3}{5}=-\frac{1*5}{4*5}+\frac{3*4}{5*4}=-\frac{5}{20}+\frac{12}{20}=\frac{12-5}{20}=\frac{7}{20}$$
2. $$\frac{9}{11}+(-\frac{2}{5})=\frac{9*5}{11*5}-\frac{2*11}{5*11}=\frac{45}{55}-\frac{22}{55}=\frac{45-22}{55}=\frac{23}{55}$$
3. $$\frac{20}{21}+\frac{3}{7}=\frac{20}{21}+\frac{3*3}{7*3}=\frac{20}{21}+\frac{9}{21}=\frac{20+9}{21}=\frac{29}{21}=1\frac{8}{21}$$
4. $$-4\frac{5}{9}+(-\frac{7}{6})=-(4\frac{5}{9}+\frac{7}{6})=-(4\frac{5*2}{9*2}+\frac{7*3}{6*3})=-(4\frac{10}{18}+\frac{21}{18})=-(4+\frac{10+21}{18})=-(4+\frac{31}{18})=-(4+1\frac{13}{18})=-5\frac{13}{18}$$
5. $$7\frac{5}{12}+(-\frac{3}{24})=7\frac{5*2}{12*2}-\frac{3}{24}=7\frac{10}{24}-\frac{3}{24}=7\frac{10-3}{24}=7\frac{7}{24}$$
6. $$-6\frac{11}{12}+(-\frac{8}{18})=-(6\frac{11}{12}+\frac{8}{18})=-(6\frac{11*3}{12*3}+\frac{8*2}{18*2})=-(6\frac{33}{36}+\frac{16}{36})=-(6+\frac{33+16}{36})=-(6+\frac{49}{36})=-(6+1\frac{13}{36})=-7\frac{13}{36}$$
7. $$-13\frac{3}{16}+7\frac{3}{16}=-13+7+\frac{3}{16}-\frac{3}{16}=-6$$
8. $$-2\frac{3}{8}+(-1\frac{5}{9})=-(2\frac{3}{8}+1\frac{5}{9})=-(2\frac{3*9}{8*9}+1\frac{5*8}{9*8})=-(2\frac{27}{72}+1\frac{40}{72})=-(3+\frac{27+40}{72})=-(3+\frac{67}{72})=-3\frac{67}{72}$$
9. $$-3\frac{1}{12}+\frac{1}{6}=-3\frac{1}{12}+\frac{1*2}{6*2}=-3\frac{1}{12}+\frac{2}{12}=-3+\frac{2}{12}-\frac{1}{12}=-3+\frac{2-1}{12}=-3+\frac{1}{12}=-2\frac{11}{12}$$
10. $$3\frac{6}{7}+(-6\frac{4}{9})=3\frac{6}{7}-6\frac{4}{9}=3\frac{6*9}{7*9}-6\frac{4*7}{9*7}=3\frac{54}{63}-6\frac{28}{63}=3-6+\frac{54}{63}-\frac{28}{63}=-3+\frac{54-28}{63}=-3+\frac{26}{63}=-2\frac{37}{63}$$
11. $$9\frac{1}{6}+(-\frac{5}{4})=9\frac{1}{6}-\frac{5}{4}=9\frac{1*2}{6*2}-\frac{5*3}{4*3}=9\frac{2}{12}-\frac{15}{12}=9+\frac{2}{12}-\frac{15}{12}=9-\frac{13}{12}=9-1\frac{1}{12}=8-\frac{1}{12}=7\frac{11}{12}$$
12. $$4\frac{3}{7}+(-\frac{8}{14})=4\frac{3}{7}-\frac{8}{14}=4\frac{3*2}{7*2}-\frac{8}{14}=4\frac{6}{14}-\frac{8}{14}=4+\frac{6}{14}-\frac{8}{14}=4-\frac{2}{14}=4-\frac{1}{7}=3\frac{6}{7}$$

Ответ: 1) $$\frac{7}{20}$$; 2) $$\frac{23}{55}$$; 3) $$1\frac{8}{21}$$; 4) $$-5\frac{13}{18}$$; 5) $$7\frac{7}{24}$$; 6) $$-7\frac{13}{36}$$; 7) $$-6$$; 8) $$-3\frac{67}{72}$$; 9) $$-2\frac{11}{12}$$; 10) $$-2\frac{37}{63}$$; 11) $$7\frac{11}{12}$$; 12) $$3\frac{6}{7}$$.