In this pattern, 8+11= 8+11 + 77 (the previous answer) = 96
As you go to the next step in the pattern, you add the previous answer as well, so:
1+4=5
2+5= 2+5+5= 12
3+6= 3+6+12 = 21
4+7= 4+7+21 = 32
5+8= 5+8+32 = 45
6+9= 6+9+45 = 60
7+10= 7+10+60 = 77
So 8+11= 8+11+77 +96
I'm not sure how to write this mathematically as such, only in a logical way, but I hope it helps!