Help #5 ​

5. The first  answer is correct, NSS....to see why this is so....note that we have

 2(1) + 2(2) + 2(3) + 2(4) + ........=

2  + 4  +   6  + 8  + .....

6.  Note that the first term is

5(3)  = 15

And the last term is  

5(8)  = 40

And the number of terms is  [ 8 - 3] + 1   = 6

So....the sum is

 [ First term + Last term ] *  number of terms / 2   =

[ 15 + 40 ] * 6 / 2   =

55 * 3   =

165