If |x|<1 and y=1+x+x2+x3+...., then write the value of dydx.
We have
y=1+x+x2+x3+....,
=(1−x)−1
dydx=ddx(1−x)−1
=(−1)(1−x)−2×(−1)
=(1−x)−2
=1(1−x)2