5. 1 4 9 16 _ _ 49 64 81

This sequence consists of perfect squares: 1^2, 2^2, 3^2, 4^2.
The missing numbers are 5^2 = 25 and 6^2 = 36.
The sequence continues with 7^2 = 49, 8^2 = 64, 9^2 = 81.

Answer: 25 36