Bạn xem hình nha 

Do mình đang gấp nên ko làm dc ạ 

$a) M=1-\frac{1}{3}=\frac{2}{3}\\ N=\frac{1}{3}-\frac{1}{5}=\frac{5}{15}-\frac{3}{15}=\frac{2}{15}\\P=\frac{1}{5}-\frac{1}{7}=\frac{7}{35}-\frac{5}{35}=\frac{2}{35}\\b) M+N=\frac{2}{3}+\frac{2}{15}=\frac{10}{15}+\frac{2}{15}=\frac{12}{15}\\M+N+P=\frac{12}{15}+\frac{2}{35}=\frac{4}{5}+\frac{2}{35}=\frac{30}{35}=\frac{6}{7}\\c) E=\frac{1}{1.3}+\frac{1}{3.5}+\frac{1}{5.7}+...+\frac{1}{19.21}\\E=\frac{1}{2}\left ( \frac{2}{1.3}+\frac{2}{3.5}+\frac{2}{5.7}+...+\frac{2}{19.21} \right )\\E=\frac{1}{2}\left ( 1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+...+\frac{1}{19}-\frac{1}{21} \right )\\E=\frac{1}{2}\left ( 1-\frac{1}{21} \right )\\E=\frac{1}{2}.\frac{20}{21}=\frac{10}{21}\\F=\frac{1}{99}-\frac{1}{99.97}-\frac{1}{97.95}-...-\frac{1}{5.3}-\frac{1}{3.1}\\F = \frac{1}{{99}} - \left( {\frac{1}{{1.2}} + \frac{1}{{2.3}} + ... + \frac{1}{{98.99}}} \right)\\  F= \frac{1}{{99}} - \left( {1 - \frac{1}{2} + \frac{1}{2} - \frac{1}{3} + ... + \frac{1}{{98}} - \frac{1}{{99}}} \right)\\ F = \frac{1}{{99}} - \left( {1 - \frac{1}{{99}}} \right)\\  F= \frac{1}{{99}} - 1 + \frac{1}{{99}}\\  F= \frac{2}{{99}} - 1 \\F= \frac{{2 - 99}}{{99}} =  - \frac{{97}}{{99}}$