1/(1.3) + 1/(3.5) + ... + 1/[(2x - 1)(2x + 1)] = 49/99
1/2 . [1 - 1/3 + 1/3 - 1/5 + ... + 1/(2x - 1) - 1/(2x + 1)] = 49/99
1/2 . [1 - 1/(2x + 1)] = 49/99
1 - 1/(2x + 1) = 49/99 . 2
1 - 1/(2x + 1) = 98/99
1 - 1/(2x + 1) = 1 - 1/99
1/(2x + 1) = 1/99
2x + 1 = 99
2x = 99 - 1
2x = 98
x = 98 : 2
x = 49