∫esecx(secxtanxf(x)+(secxtanx+sec2x)dx=esecxf(x)+C
Differentiating both sides w.r.t ‘x’ we get
esecx(secxtanxf(x)+(secxtanx+sec2x))=(esecx⋅secxtanx)f(x)+esecxf′(x)
⇒f′(x)=sec2x+tanxsecx
⇒f(x)=∫(sec2x+tanxsecx)dx
⇒f(x)=tanx+secx+c,c∈R
Hence, possible choice is f(x)=secx+tanx+21.