We know that,
color(red)((1)cot^-1(-x)=pi-cot^-1x(1)cot−1(−x)=π−cot−1x
color(red)((2)sec(pi-theta)=-sectheta(2)sec(π−θ)=−secθ
color(red)((3)cot^-1x=tan^-1(1/x), x > 0(3)cot−1x=tan−1(1x),x>0
color(red)((4)tan^-1x=cos^-1(1/(sqrt(1+x^2)))(4)tan−1x=cos−1(1√1+x2)
color(red)((5)cos^-1x=sec^-1(1/x)(5)cos−1x=sec−1(1x)
color(red)((6)sec(sec^-1x)=x(6)sec(sec−1x)=x
Here,
sec(cot^-1(-6))=sec(pi-cot^-1(6))...toApply color(red)((1)
=-sec(cot^-1(6)).......toApply color(red)((2)
=-sec(tan^-1(1/6)).........toApply color(red)((3)
=-sec(cos^-1(1/(sqrt(1+(1/6)^2))))......toApply color(red)((4)
=-sec(cos^-1(1/(sqrt(1+1/36))))
=-sec(cos^-1(6/(sqrt(37))))
=-sec(sec^-1(sqrt(37)/6)).......to Apply color(red)((5)
=-sqrt(37)/6...........toApply color(red)((6)