Question

[Lê Nam Khánh0103]

tính tổng A=\(\frac{3}{1.3}\)+\(\frac{3}{3.5}\)+ \(\frac{3}{5.7}\)+........+\(\frac{3}{49.51}\)

Answer 1

[Lê Trần Ngọc Hằng]

ta có A=3/1*3+3/3*5+3/5*7+...+3/49*51

=> A=3*1/2*(2/1*3+2/3*5+..+2/49*51)

=> A=3/2*(1-1/3+1/3-1/5+..+1/49-1/51)

=> A=3/2*(1-1/51)

=> A= 3/2* 50/51

=> A= 25/17