Question

[kiwi nguyễn]

CMR:

a) \(\frac{1}{3}+\frac{1}{3^2}+\frac{1}{3^3}+...+\frac{1}{3^{99}}< \frac{1}{2}\)

b) \(\frac{3}{1^2.2^2}+\frac{5}{2^2.3^2}+\frac{7}{3^2.4^2}+...+\frac{19}{9^2.10^2}< 1\)

Answer 1

[Minh Linh Dam Duc]

a)Xét vế trái , ta có :

Gọi tổng các số hạng ở vế trái là A

=> A= \(\frac{1}{3}\)+\(\frac{1}{3^2}\)+ ... +\(\frac{1}{3^{99}}\)

=>3A = 1 + \(\frac{1}{3}\)+ \(\frac{1}{3^2}\)+ ... + \(\frac{1}{3^{98}}\)

=> 3A - A = 1 + \(\frac{1}{3}\)+ \(\frac{1}{3^2}\)+ ... + \(\frac{1}{3^{98}}\)- ( \(\frac{1}{3}\)+\(\frac{1}{3^2}\)+ ... +\(\frac{1}{3^{99}}\))

=> 2A = 1 - \(\frac{1}{3^{99}}\)

=> A = \(\frac{1}{2}\)- \(\frac{1}{3^{99}.2}\) < \(\frac{1}{2}\)

b)\(\frac{3}{1^2.2^2}\)+ \(\frac{5}{2^2.3^2}\)+ ... + \(\frac{19}{9^2.10^2}\)

= \(\frac{3}{1.4}\)+ \(\frac{5}{4.9}\)+ .... + \(\frac{19}{81.100}\)

= 1 - \(\frac{1}{4}\)+ \(\frac{1}{4}\)- \(\frac{1}{9}\)+ ... + \(\frac{1}{81}\)- \(\frac{1}{100}\)

= 1 - \(\frac{1}{100}\) <1

Answer 2

[Ngọc Lan Tiên Tử]

a,

\(\sum\limits^{99}_{x=1}\left(\frac{1}{3^x}\right)=\frac{1}{2}\)

bài a nó có ............