Question

1,3,9,31,?,651 what no. Will come in place of question mark??

Answer 1

Given sequence is 1, 3, 9, 31, ?, 651

First term = 1
Second term = 1*1 + 2 = 3
Third term = 3*2 + 3  = 9
Fourth term = 9*3 + 4 = 31
Fifth term = 31*4 + 5 = 129
Sixth term = 129*5 + 6 = 651

The number in place of "?" is 129

Answer 2

Given sequence is 1,3,9,31,?,651
1 * 1 + 2 = 3
3 * 2 + 3 = 9
9 * 3 *+ 4 = 31
31 * 4 + 5 = ?
124 + 5 = 129
129 * 5 + 6 = 651
129 will come in the place of question mark.