5/21+4/9; -5/21-4/9; 5/21-4/9; -5/21+4/9

Решите задачу:

$\frac{5}{21} + \frac{4}{9} = \frac{5*3}{21*3} + \frac{4*7}{9*7} = \frac{15+28}{63} = \frac{43}{63}$

$- \frac{5}{21}- \frac{4}{9} =- \frac{5*3}{21*3} - \frac{4*7}{9*7} =- \frac{43}{63}$

$\frac{5}{21}- \frac{4}{9} = \frac{5*3}{21*3}- \frac{4*7}{9*7} = \frac{15-28}{63} = \frac{-13}{63}$

$- \frac{5}{21} + \frac{4}{9} = \frac{4}{9} - \frac{5}{21} = \frac{4*7}{9*7} - \frac{5*3}{21*3} = \frac{28-15}{63} = \frac{13}{63}$