If 1 + 4 = 5, 2+ 5 = 7, & 3 + 6 = 21, then what is 8 + 11 ?

It's actually: If 1+4=5, 2+5=12, 3+6=21, 8+11=?
And the answer is: 8+11=40
Because:
1+4=5 (Take that result and add it the the result of the next equation)
(5)+2+5=12 (Repeat)
(12)+3+6=21 (Repeat)
(21)+8+11=40