To find the sum of the given series, we need to find the sum of each term and then add them all together.

The general term of the series can be written as (2n-1)(2n+1), where n is the position of the term.

For the first term, n = 1:
(2(1)-1)(2(1)+1) = 1(3) = 3

For the second term, n = 2:
(2(2)-1)(2(2)+1) = 3(5) = 15

Continuing this pattern, we can find the sum of each term up to the last term, n = 49:
(2(49)-1)(2(49)+1) = 97(101) = 9797

Now, we can find the sum of the series by adding up all the terms:
3 + 15 + 35 + ... + 9797

To find the sum of an arithmetic series, we can use the formula Sn = (n/2)(a + l), where Sn is the sum of the series, n is the number of terms, a is the first term, and l is the last term.

In this case, n = 49, a = 3, and l = 9797.

Sn = (49/2)(3 + 9797)
Sn = (49/2)(9800)
Sn = 49(4900)
Sn = 240100

Therefore, the sum of the series 1x3 + 5x7 + ... + 97x101 is 240100.