00:01
Hello everyone, this is the first part they have asked the radius and interval of convergence of the power series summation n equal to 0 to infinity n square by 2 to the power n x n.
00:12
So for this we use a ratio test.
00:17
So the ratio test tells that limit n tends to infinity modulus of a n plus 1 by a n equal to l.
00:26
So if l less than 1 the series converges.
00:33
So first we write a n a n is nothing but n square by 2 to the power n x n a n plus 1 is n plus 1 the whole square 2 to the power n plus 1 x to the power n plus.
00:49
So a n plus 1 by a n will be n plus 1 the whole square by 2 to the power n plus 1 x to the power n plus 1 divided by n square by 2 to the power n x n.
01:03
So we will be having n plus 1 the whole square x by 2 divided by n square that is this 2 to the power n plus 1 here 2 to the power n and 2 to the power n will get cancelled.
01:22
Similarly x to the power n x to the power n will get cancelled.
01:25
So we have left with x 2 and n square.
01:29
So this can be written as a n plus 1 by n as n plus 1 the whole square by 2 n square x.
01:40
This further can be written as a n plus 1 by a n as n plus 1 by n the whole square by 2 x.
01:54
For taking limit i have written like this.
01:57
So now we can take the limit before taking the limit.
02:02
This can be further written as that is a n plus 1 by a n as taking 1 by 2 and n by n will give you 1 1 plus 1 by n the whole square x.
02:15
So this is the final a n plus 1 divided by a n.
02:19
Now we apply the limit...