Solving the recurrence of recursive insertion sort
Why do u need to guess?
T(n) = T(n-1) + cn  {c is a const}
T(n) = T(n-2) +c(n-1) +cn
     = T(n-3) +c*[(n-2)+ (n-1) + n]
..... 

Keep going this way till u reach the arithmetic sequence  (2+...+(n-1)+n]. So, 

T(n) = T(1) + c* n(n-1)/2 = O(n2) 

You may check any Algorithms book or lecture notes on solving recursive equations; we don't usually guess, there r rules and this one is trivial.