Question

how to did it become a+b​

Answer 1

Answer:

No answer for me. Very sorry.

Answer 2

How it became a+b is proved by ratios.

Given that : $\frac{b^2(a+b)}{y} =\frac{a+b}{1}$

Multiply $\frac{1}{a+b}$ to both sides, we get

⇒ $\frac{b^2}{y}=\frac{a+b}{a+b} =1$

Hence the expression is true.