1 plus 4 equals 5 riddle

1+4=5

2+5=12

3+6=21

5+8=?

Answer: Please provide your opinion below in comments section

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Logic used is a+a(b) 

1 +1(4) = 5

2+2(5)=12

3+3(6)=21

So,5+5(8)=45.

Answer is 45.