a)B=($\frac{1}{10}$-1).($\frac{1}{11}$-1)...($\frac{1}{100}$-1)
B=$\frac{-9}{10}$.$\frac{-10}{11}$...$\frac{-99}{100}$
B=$\frac{-(9.10...99)}{10.11....100}$
B=$\frac{-9}{100}$
b)E=(1-$\frac{1}{2^{2}}$).(1-$\frac{1}{3^{2}}$)...(1-$\frac{1}{20^{2}}$)
E=$\frac{1.3}{2^{2}}$.$\frac{2.4}{3^{2}}$....$\frac{9.11}{10^{2}}$
E=$\frac{1.2...9}{2.3....10}$ .$\frac{3.4...11}{2.3....10}$
E=$\frac{1}{10}$.$\frac{11}{2}$
E=$\frac{11}{20}$
Mik chỉ làm đc vậy thui sr bạn :(
