a) \(sin\left(x+\dfrac{\pi}{2}\right)=cos\left[\dfrac{\pi}{2}-\left(x+\dfrac{\pi}{2}\right)\right]=cos\left(-x\right)=cosx\) a : Đúng. b) \(cos\left(x+\dfrac{\pi}{2}\right)=sin\left[\dfrac{\pi}{2}-\left(x+\dfrac{\pi}{2}\right)\right]=sin\left(-x\right)=-cosx\) b: Sai. c) \(sin\left(x-\pi\right)=-sin\left(\pi-x\right)=-sinx\). d: Sai. d) \(cos\left(x-\pi\right)=cos\left(\pi-x\right)=cosx\) c: Đúng.