It is not "putting forward the upper limit of integral" or "putting forward the factor of upper limit of integral". But:

I =∫& lt； 0，π& gt； (sinx)^4dx =∫& lt； 0，π/2 & gt； (sinx)^4dx+∫& lt； π/2，π& gt； (sinx)^4dx

For the latter, let x = π-u, and we get

I2 =∫& lt； π/2，π& gt； (sinx)^4dx =∫& lt； π/2，0 & gt(sinu)^4(-du)=∫& lt； 0，π/2 & gt； (sinu)^4du

Definite integral has nothing to do with integral variable. If u is replaced by x, I2 = ∫.

Then I = ∫ < 0, π >; (sinx)^4dx = 2∫& lt； 0，π/2 & gt； (sinx)^4dx