Which of the following is/are correct?

a.Let f(x)=excos⁡x−1⇒f′(x)=ex(cos⁡x−sin⁡x)=0⇒tan⁡x=1, which has a root between two roots of f(x)=0if fx has two roots then f1x has a root between them by Rolles mean value theorem  b. Let f(x)=exsin⁡x−1,f′(x)=ex(sin⁡x+cos⁡x)=0⇒tan⁡x=−1, which has a root between two roots of f(x)=0 c. Let f(x)=e−x−cos⁡xf′(x)=−e−x+sin⁡x=0⇒e−x=sin⁡x, which has a root between two roots of f(x)=1