Each term in the series is the product of the digits of the preceding term.

So, missing term = 1 x 8 = 8.

Clearly, 3 + 6 = 9, 9 + 6 = 15,.....

So, the series is an A.P. in which a = 3 and d = 6.

Therefore 21st term = a + (21 - 1) d = a + 20d = 3 + 20 x 6 = 123.

The pattern is + 5, - 2, + 5, - 2,.....

So, missing term = 36 - 2 = 34.

The series consists of squares of consecutive odd numbers

i.e. 1², 3², 5², 7², 9²,.....

So, missing term = 11² = 121.

The given series consists of squares of consecutive odd numbers

i.e. 1², 3², 5², 7²,.....

So, missing term = 9² = 81.