SEAMO 2027 - Paper F - SET 10 - Self-study Question 15

In the expansion of $f(x)=(1+ax)^4(1+bx)^5$, where $a,b$ are positive integers, the coefficient of $x^2$ is 66. Evaluate $a+b$.

Source-consistency warning: the printed numerical data and supplied answer key are inconsistent. Retain the expression and all data exactly as printed, and assess the listed choices from those data.