# By using the Ratio Test determine if the series converges: `Sigma` `(-1)^n/4^n`

### 3 Answers | Add Yours

This is an alternating series. If alternating series is absolutely convergent (series of absolute values is convergent) then the alternating series is convergent` `

`sum |a_n|->a=>sum a_n->b,` `a,b in RR`

Also if `lim_(n->oo)|(a_(n+1))/(a_n)|=q` then the series is convergent if `q<1` and divergent if `q>1.`

Let's now check convergence of our series.

`lim_(n->oo)|((-1)^(n+1))/(4^(n+1))/((-1)^n)/(4^n)|=lim_(n->oo)4^n/4^(n+1)=`

`lim_(n->oo)4^n/(4cdot4^n)=lim_(n->oo)1/4=1/4`

**Since `1/4<1` it follows that the series is convergent.**

The series `sum_(n=1)^ooa_n` where `a_n` is non zero is

(i) convergent if the ratio `lim_(n->oo)mod(a_(n+1)/a_n)<1`

(ii)divergent if the ratio `lim_(n->oo)mod(a_(n+1)/a_n)>1` .

Here `a_n=(-1)^n/4^n` , `a_(n+1)=(-1)^(n+1)/4^(n+1)`

Now `a_(n+1)/a_n={(-1)^(n+1)/4^(n+1)}/{(-1)^n/4^n}` =-1/4

Now `lim_(n->oo)mod (a_(n+1)/a_n)=1/4<1`

So the given series is convergent.

By using The Ratio Test determine if the series converges:

`sum` `(-1)^n/4^n`