Question

1.Tính bằng phương pháp hợp lí

a) A=\(\frac{3}{4.7}\)+\(\frac{3}{7.10}\)+...+\(\frac{3}{73.76}\)

b) B=\(\frac{5}{10.11}\)+\(\frac{5}{11.12}\)+...+\(\frac{5}{99.100}\)

Dấu (.) là dấu nhân nha

Answer 1

A=3/4.7+3/7.10+...+3/73.76

A=1/4-1/7+1/7-1/10+1/10-1/13+....+1/73-1/76

A=1/4-1/76

A=9/38

b)  B=5/10.11+5/11.12+....+5/99.100

       B=5(1/10.11+1/11.12+1/12.13+...+1/99.100)

       B=5(1/10-1/11+1/11-1/12+1/12-1/13+...+1/99-1/100)

       B=5(1/10-1/100)

       B=5.99/100

      B=99/20

Answer 2

\(\frac{3}{4.7}+\frac{3}{7.10}+...+\frac{3}{73.76}\)

=\(\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+...+\frac{1}{73}-\frac{1}{76}\)

=\(\frac{1}{4}-\frac{1}{76}\)

=\(\frac{9}{38}\)