A = \(\dfrac{1}{4}\) + \(\dfrac{1}{16}\) + \(\dfrac{1}{36}\) +...+ \(\dfrac{1}{196}\)
A = \(\dfrac{1}{2^2}\) + \(\dfrac{1}{4^2}\) + \(\dfrac{1}{6^2}\)+...+ \(\dfrac{1}{14^2}\)
A = \(\dfrac{1}{\left(1.2\right)^2}\) + \(\dfrac{1}{\left(2.2\right)^2}\) + \(\dfrac{1}{\left(2.3\right)^2}\)+...+ \(\dfrac{1}{\left(2.7\right)^2}\)
A = \(\dfrac{1}{1^2.2^2}\) + \(\dfrac{1}{2^2.2^2}\)+ \(\dfrac{1}{2^2.3^2}\)+...+ \(\dfrac{1}{2^2.7^2}\)
A = \(\dfrac{1}{2^2}\) \(\times\)( \(\dfrac{1}{1}\) + \(\dfrac{1}{2^2}\) + \(\dfrac{1}{3^2}\)+...+ \(\dfrac{1}{7^2}\))
Vì \(\dfrac{1}{2}>\dfrac{1}{3}>\dfrac{1}{4}>\dfrac{1}{5}\) \(>\)\(\dfrac{1}{6}>\dfrac{1}{7}\)
⇒ \(\dfrac{1}{2.2}\)+\(\dfrac{1}{3.3}\)+\(\dfrac{1}{4.4}\)+\(\dfrac{1}{5.5}\)+\(\dfrac{1}{6.6}\)+\(\dfrac{1}{7.7}\) < \(\dfrac{1}{1.2}\)+\(\dfrac{1}{2.3}\)+\(\dfrac{1}{3.4}\)+\(\dfrac{1}{4.5}\)+\(\dfrac{1}{5.6}\)+\(\dfrac{1}{6.7}\)
⇒ A < \(\dfrac{1}{2^2}\) \(\times\) ( 1 + \(\dfrac{1}{1}\) - \(\dfrac{1}{2}\) + \(\dfrac{1}{2}\) - \(\dfrac{1}{3}\)+ \(\dfrac{1}{3}\) - \(\dfrac{1}{4}\) + \(\dfrac{1}{4}\) - \(\dfrac{1}{5}\) + \(\dfrac{1}{5}\) - \(\dfrac{1}{6}\) + \(\dfrac{1}{6}\) - \(\dfrac{1}{7}\))
⇒ A < \(\dfrac{1}{4}\) \(\times\) ( 2 - \(\dfrac{1}{7}\))
⇒ A < \(\dfrac{1}{2}\) - \(\dfrac{1}{28}\) < \(\dfrac{1}{2}\)
⇒ A < \(\dfrac{1}{2}\) ( đpcm)