Question

[HoàngNgọcPhonh]

Giải giúp mình gấp bài này với: S= \(-\frac{3^2}{4}\) -  \(\frac{3^2}{28}\) - \(\frac{3^2}{70}\) - ...- \(\frac{3^2}{868}\) giải gấp giúp mình nha

Answer 1

[Nguyễn Hữu Hoàng Hải Anh]

S= - 3²\(\left(\frac{1}{4}+\frac{1}{28}+\frac{1}{70}+...+\frac{1}{868}\right)\)

S = - 3²\(\left(\frac{1}{1.4}+\frac{1}{4.7}+\frac{1}{7.10}+...+\frac{1}{28.31}\right)\)

S = - 3\(\left(\frac{3}{1.4}+\frac{3}{4.7}+...+\frac{3}{28.31}\right)\)

S = -3\(\left(1-\frac{1}{4}+\frac{1}{4}-\frac{1}{7}+...+\frac{1}{28}-\frac{1}{31}\right)\)

S = -3 \(\left(1-\frac{1}{31}\right)\)

S = -3\(.\frac{30}{31}\)

S = -90/31

Answer 2

[nguyen xuan mai]

1/3S=-(1/1*4+1/4*7+1/7*10+...+1/28*31)=-(1/1-1/4+1/4-1/7+1/7-1/10+...+1/28-1/31)=-(1/1-1/31)=-30/31

=>S=(-30/31):1/3=-90/31