In Summation We can use the Merge Role :
So We Can Merge Like Below :
when i develop the series of a cosine i have a (-1) member
i wanted to represent the series as a sum
so i need to take only the odd members so the power of -1 is 2k+1 i got
but the solution says that the power of -1 is equal
is it the same??
why they have such an expression
(they use n istead of k)
how they got the power?