Tan(x)= sin(x)/cos(x), therefore, substitute sin(x)/cos(x) in the expression: =(cos(x)(sin(x)/cos(x))-1)/cos^2(x) simplify the 2 cos(x): =(sin(x)-1)/cos^2(x) Sin^2(x)+ cos^2(x)=1, sin^2(x)-1=- cos^2(x), substitute in the expression: = -cos^2(x)/cos^2(x)= -1
