Действия с обыкновенными дробями

$$\frac{4}{25}+\frac{15}{4}=$$$$\frac{4*4+15*25}{4*25}=$$$$\frac{391}{100}=3,91$$

$$\frac{3}{2}-\frac{9}{5}=$$$$\frac{3*5-9*2}{2*5}=$$$$\frac{15-18}{10}=-0,3$$

$$\frac{3}{4}*\frac{6}{5}=$$$$\frac{3*6}{4*5}=$$$$\frac{3*3}{2*5}=0,9$$

$$\frac{3}{5}:\frac{2}{15}=$$$$\frac{3}{5}*\frac{15}{2}=$$$$\frac{3*15}{5*2}=4,5$$

$$\frac{1}{4}+0,7=$$$$\frac{1}{4}+\frac{7}{10}=$$$$\frac{1}{2*2}+\frac{7}{2*5}=$$$$\frac{1*5+7*2}{2*2*5}=$$$$\frac{19}{20}=0,95$$

$$\frac{0,9}{1+\frac{1}{8}}=$$$$\frac{\frac{9}{10}}{\frac{9}{8}}=$$$$\frac{9}{10}*\frac{8}{9}=0,8$$

$$\frac{1}{\frac{1}{18}-\frac{1}{21}}=$$$$\frac{1}{\frac{1}{6*3}-\frac{1}{7*3}}=$$$$\frac{1}{\frac{7}{3*6*7}-\frac{6}{3*6*7}}=$$$$\frac{1}{\frac{7-6}{3*6*7}}=$$$$\frac{3*6*7}{1}=126$$

$$(\frac{19}{8}+\frac{11}{12})\div \frac{5}{48}=$$$$\frac{19*3+11*2}{24} * \frac{48}{5}=$$$$\frac{57+22}{24}*\frac{48}{5}=$$$$\frac{79*2}{5}=31,6$$

$$1\frac{8}{17}:(\frac{12}{17}+2\frac{7}{11})=$$$$\frac{8+17*1}{17}:(\frac{12}{17}+\frac{11*2+7}{11})=$$$$\frac{25}{17}:(\frac{12}{17}+\frac{29}{11})=$$$$\frac{25}{17}:\frac{12*11+29*17}{11*17}=$$$$\frac{25}{17}*\frac{11*17}{625}=$$$$\frac{25*11}{625}=\frac{11}{25}=0,44$$

$$(\frac{1}{13}-2\frac{3}{4})\cdot 26=$$$$(\frac{1}{13}-\frac{11}{4})\cdot 26=$$$$\frac{4*1-11*13}{13*4}\cdot 26=$$$$\frac{-139*26}{13*4}=$$$$-\frac{139}{2}=-69,5$$

$$18*(\frac{1}{2})^2-22,5*\frac{1}{9}=$$$$18*\frac{1}{4}-\frac{22,5}{9}=$$$$\frac{18}{4}-\frac{22,5}{9}=$$$$\frac{18*9-22,5*4}{4*9}=$$$$\frac{162-90}{36}=2$$

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$$(\frac{2}{3}+\frac{5}{6})\cdot(3\frac{1}{8}-\frac{5}{12})=$$ $$\frac{9}{6}(\frac{75}{24}-\frac{10}{24})=$$ $$\frac{9}{6}\cdot\frac{65}{24}=\frac{65}{16}=4,0625$$

Воспользуемся свойствами обыкновенных дробей: $$(\frac{32}{38}-\frac{4}{19}):(\frac{9}{38}+\frac{7}{38})=$$$$(\frac{16}{19}-\frac{4}{19}):\frac{9+7}{38}=$$$$\frac{16-4}{19}*\frac{38}{16}=$$$$\frac{12*38}{19*16}=\frac{3}{2}=1,5$$

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$$(\frac{9}{16}+\frac{5}{18}):(2\frac{3}{4})^{2}-3\frac{1}{9}=$$$$\frac{81+40}{2*8*9}*(\frac{4}{11})^{2}-\frac{28}{9}=$$$$\frac{121}{16*9}*\frac{16}{121}-\frac{28}{9}=$$$$\frac{1}{9}-\frac{28}{9}=-3$$

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$$18*(\frac{2}{3})^{2}-8*(2\frac{1}{2})^{2}=$$$$18*\frac{4}{9}-8*(\frac{5}{2})^{2}=$$$$8-8*\frac{25}{4}=8-50=-42$$