Question

[Phạm Trần Ngọc Diệp]

\(D=1+\dfrac{-1}{20}+\dfrac{-1}{30}+\dfrac{-1}{42}+\dfrac{-1}{56}+\dfrac{-1}{72}+\dfrac{-1}{90}\)

Answer 1

[Nguyễn Thị Thương Hoài]

D = 1 + \(\dfrac{-1}{20}\) + \(\dfrac{-1}{30}\) + \(\dfrac{-1}{42}\)+ \(\dfrac{-1}{56}\)+ \(\dfrac{-1}{72}\)+ \(\dfrac{-1}{90}\)

D = 1 - ( \(\dfrac{1}{4\times5}\) + \(\dfrac{1}{5\times6}\)+ \(\dfrac{1}{6\times7}\)+ \(\dfrac{1}{7\times8}\)+\(\dfrac{1}{8\times9}\)+\(\dfrac{1}{9\times10}\))

D = 1 - ( \(\dfrac{1}{4}\) - \(\dfrac{1}{10}\))

D = 1 - \(\dfrac{3}{20}\)

D = \(\dfrac{17}{20}\)

Answer 2

[ma thanh ngan]

D=1+(1/4x5+1/5x6+1/6x7+1/7x8+1/8x9+1/9x10)

D=1+(1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9+1/9-1/10)

D=1+(1/4-1/10)

D=1+3/5

D=8/5