1^2/1x2 x 2^2/2x3 ......... 17^2/17x18 x 18^2/18x19 cứu mik với !

$\frac{1^2}{1.2} .  \frac{2^2}{2.3}...\frac{17^2}{17.18}.\frac{18^2}{18.19}$

$ = \frac{1^2.2^2...17^2.18^2}{(1.2).(2.3)...(17.18).(18.19)}$

$ = \frac{1.2...17.18}{1.2...17.18} . \frac{1.2...17.18}{2.3...18.19}$

$ = 1. \frac{1}{19}$

$ = \frac{1}{19}$