Question

When a lateral cylinder is cut, we get a rectangle. Why is the formula for curved surface area of cylinder is not l*b ?

Answer 1

Lateral Surface Area (LSA or CSA) = 2π r h

When we have a cylinder we dont have its length or breadth at first.
The only parameter we have is its radius, and height.
We need to find the area of cylinder with that two variables.
So we use the above equation to find the area.

The length and breadth are obtained only when we cut the cylinder.
That is not possible. So the above formula is being used.
Hope it helps.