# Thread: my teacher marked this wrong on the test

1. ## my teacher marked this wrong on the test

should i argue with her, or is this just wrong.

i got 0 credit for it, instructions of the test were:
"fore each of the follwoing, determine wheter the infinite series converges or diverges. if it is an alternating series, determine whether it coverges conditionally or absolutely.

is that mathmatically wrong somewhere?

2. Originally Posted by Legendsn3verdie
should i argue with her, or is this just wrong.

i got 0 credit for it, instructions of the test were:
"fore each of the follwoing, determine wheter the infinite series converges or diverges. if it is an alternating series, determine whether it coverges conditionally or absolutely.

is that mathmatically wrong somewhere?
Yes it's wrong. Your teacher is quite correct to give you zero credit for this solution. It shows a fundamental lack of understanding of series algebra.

The result of the nth root test is 2/5, NOT 5*(2/5).

Once you take the 5 out of the summation, your $\displaystyle a_n$ is $\displaystyle \left(\frac{2}{5}\right)^n$.

Once the 5 is taken out, all it does is multiply with the value that the series converges to.

3. Originally Posted by Legendsn3verdie
should i argue with her, or is this just wrong.

i got 0 credit for it, instructions of the test were:
"fore each of the follwoing, determine wheter the infinite series converges or diverges. if it is an alternating series, determine whether it coverges conditionally or absolutely.

is that mathmatically wrong somewhere?
The n-th root of the n-th term is $\displaystyle \sqrt[n]{5 (2/5)^n}$ not $\displaystyle 5 \sqrt[n]{(2/5)^n}$

Alternativly you should be applying the n-th root test to the series $\displaystyle \sum_{n=1}^{\infty}(2/5)^n$

CB

4. hm yah ok i guess i was completly wrong thanks for helpin me understand my error.