In what situation do we need to use substitution rule? what is the purpose for using substitution rule?

Substitution rule is one of the most important rule to solve the integrals problem.

In substitution rule we substitute  different  variable in such a way that it can simplify the question.

example: when we have to solve "Sin x²" we cannot solve it but

               if question is "x  Sin x²" we can substitute x² with t and can solve it.

The purpose of substitution rule is to simplify the problem of integrals.

it will cost dollar $540

the living room is 3 which means if we divide 325/3 then we get $108

therefore if 1part of the living room cost $108 and 3 costed $325

then total 5part of the room will cost 5*108 which gives $540