Bài 4:
a) `A= (2/7 + 2/5 + 2/17 - 2/293)/(3/7 + 3/5 + 3/17 - 3/293)`
`A= (2(1/7 + 1/5 +1/17 - 1/293))/(3(1/7 + 1/5 + 1/17 - 1/293))`
`A= 2/3`
Vậy `A= 2/3`
b) `B= 3/1.3 + 3/3.5 + 3/5.7 +...+ 3/49.51`
`B= 3(1/1.3 + 1/3.5 + 1/5.7 +...+1/49.51)`
`2B= 3. 2(1/1.3 + 1/3.5 + 1/5.7+...+1/49.51)`
`2/3 B = 2/1.3 + 2/3.5 + 2/5.7 + ...+2/49.51`
`2/3 B= 1/1 - 1/3 + 1/3 -1/5+...+1/49-1/51`
`2/3 B= 1 - 1/51`
`2/3 B= 50/51`
`B= 50/51: 2/3`
`B= 50/51. 3/2`
`B= 25/17`
Vậy `B= 25/17`
