Consider a unit circle having centre at origin. ON=PM=sin‌θ‌‌PQ=tan‌θ‌‌OQ=sec‌θ‌‌OP=1 OM=PN=cos‌θ‌‌PR=cot‌θ‌‌OR=cosecθ‌‌MN=1 ∴f(θ)=PM+PN+PQ+PR−OQ−OR =PM+PN+QR−OQ−OR =PM+PN+QR−(OM+MQ)−(ON+NR) =QR−MQ−NR <MN (Since, NR+MN+MQ>QR) ∴f(θ)<1