As, sum of first n natural number
=So, sum of first 15 natural number
∴ M = Mean of first 15 natural number
==8∴M=8[first 15 natural numbers, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]
Now, variance,
V=‌∑(xi−M)2=[280]=∴V+M2=+82=+64==