Nếu x=u(t) thì: f(x)dx = f(u(t)) . u'(t) dt

A. $f\left(x\right)dx=f\left(u\left(t\right)\right).u\text{'}\left(t\right)dt$

B. $f\left(x\right)dx=f\left(u\left(t\right)\right)dt$

C. $f\left(x\right)dx=f\left(u\text{'}\left(t\right)\right)dt$

D. $f\left(x\right)dx=f\left(u\left(t\right)\right).u\left(t\right)dt$