Question

 Вычислите : 1)3/7+4/9; 2)8/9-7/8;3)13/15-2/3;4)20/21+3/7;5) 17/18-11/12;6)7/16+1/6;7)2/9+5/6;8)10/21+9/14;9)7/9-4/15;10)9/14-3/7+15/28 11)1/6+1/4-1/8 12)13/18-29/45+8/15

Answer 1

$1)ttdisplaystylefrac{3}{7}+frac{4}{9}=frac{3*9}{7*9}+frac{4*7}{9*7}=frac{27}{63}+frac{28}{63}=frac{55}{63}$

$2)ttdisplaystylefrac{8}{9}-frac{7}{8}=frac{8*8}{9*8}-frac{7*9}{8*9}=frac{64}{72}-frac{63}{72}=frac{1}{72}$

$3)ttdisplaystylefrac{3}{15}-frac{2}{3}=frac{3}{15}-frac{2*5}{3*5}=frac{3}{15}-frac{10}{15}=-frac{7}{15}$

$4)ttdisplaystylefrac{20}{21}+frac{3}{7}=frac{20}{21}+frac{3*3}{7*3}=frac{20}{21}+frac{9}{21}=frac{29}{21}=1frac{9}{21}$

$5)ttdisplaystylefrac{17}{18}-frac{11}{12}=frac{17*2}{18*2}-frac{11*3}{12*3}=frac{34}{36}-frac{33}{36}=frac{1}{36}$

$6)ttdisplaystylefrac{7}{16}+frac{1}{6}=frac{7*3}{16*3}+frac{1*8}{6*8}=frac{21}{48}+frac{8}{48}=frac{29}{48}$

$7)ttdisplaystylefrac{2}{9}+frac{5}{6}=frac{2*2}{9*2}+frac{5*3}{6*3}=frac{4}{18}+frac{15}{18}=frac{19}{18}=1frac{1}{18}$

$8)ttdisplaystylefrac{10}{21}+frac{9}{14}=frac{10*2}{21*2}+frac{9*3}{14*3}=frac{20}{42}+frac{27}{42}=frac{47}{42}=1frac{5}{42}$

$9)ttdisplaystylefrac{7}{9}-frac{4}{15}=frac{7*5}{9*5}-frac{4*3}{15*3}=frac{35-12}{45}=frac{23}{35}$

$10)ttdisplaystylefrac{9}{14}-frac{3}{7}+frac{15}{28}=frac{9*2}{14*2}-frac{3*4}{7*4}+frac{15}{28}=frac{18-12+15}{28}=frac{21}{28}=frac{3}{4}$

$11)ttdisplaystylefrac{1}{6}+frac{1}{4}-frac{1}{8}=frac{1*4}{6*4}+frac{1*6}{4*6}-frac{1*3}{8*3}=frac{4+6-3}{24}=frac{7}{24}$

$12)ttdisplaystylefrac{13}{18}-frac{29}{45}+frac{8}{15}=frac{13*5}{18*5}-frac{29*2}{45*2}+frac{8*6}{15*6}=frac{65-58+48}{90}=frac{55}{90}=frac{11}{18}$