as Lt x→0 by keeping11tan2x+2tan2x1tan3x..∞=y then Take log on both sides
log 1y= cot2xlog(tan2x+2tan2x1tan3x..∞)
then Take cot2x as 1tan2x
Now Diffrentiate on both sides As L'Hospital rule
and use the tan Expansion formula
then You will get the answer