Question 25541
"d" is the common difference between any 2 consecutive numbers 
In this case the sequence is 1,3,5,7........
So the difference becomes 3-1= 5-3 = 7-5 = 2

Now every arithmetic series is represented by {{{a+(n-1)d}}} where a is the first term and d the common difference
n-1 is the nth term

For the given series, a = 1 since the series begins with 1 and d = 2
The 1st term is 1
2nd term is 3
So the 101st term becomes 1+(101-1)2  = 1+200 = 201

Sum to n terms is given by {{{(n/2)*First Term + Last Term}}} where n is the number of terms. 
Please solve this on your own as I have already shown you above how to find the last term. Again you can the find the sum to the first 30 terms with the same formula.

Adding the first two numbers or the first three or so on, you will find that the sum is always an Even Number.