first find nth term of given series
then apply summation
u will get
 
 
but this methd is more cmplicated for this type of problems.
so to save time use verificaion process that is u have put a natural number in place of n 
if u took n=1 then add first term of given series
if u took n=2 then add first two terms of given series mstly n=1 gives u crrect ans
						

							
i mean to say that certain options r provided to u in competitive exams
in the above problem may be 4 options  will be provided to u
definetly those options will be in terms of n
so, now u have to put n=1 in those options u will get certain answer and it will be equal to 1.2.3.4.5 as it is the first term in given series
if more than one option satisfies it,then take n=2 and it will be equal to 1.2.3.4.5+2.3.4.5.6. (it is the sum of first two terms in given series)
 
REMEMBER U MUST TAKE NATURAL NUMBERS IN THE PLACE OF n 
it is best way to solve this kind of problems.i always use this method  nd always get correct ans
for simple series u can easily apply summation method
 
hope it will also helped u 
And approve if it helped u