Question

tính tổng D= 4/11.16+4/16.21+...+4/61.66

Answer 1

D=\(\dfrac{4}{11\cdot16}\)+\(\dfrac{4}{16\cdot21}\)+...+\(\dfrac{4}{61\cdot66}\)

D=\(\dfrac{4}{5}\)(\(\dfrac{1}{11}\)-\(\dfrac{1}{16}\)+\(\dfrac{1}{16}\)-\(\dfrac{1}{21}\)+...+\(\dfrac{1}{61}\)-\(\dfrac{1}{66}\))

D=\(\dfrac{4}{5}\)(\(\dfrac{1}{11}\)-\(\dfrac{1}{66}\))

D=\(\dfrac{4}{5}\)x\(\dfrac{5}{66}\)=\(\dfrac{2}{33}\)