First, multiply by 1:
cos x * 1 + cos x tan^2 x
Factor cos x out of cos x tan^2 x
cos x * 1 + cos x (tan ^2 x)
Factor again
cos x (1+ tan^2 x)
Rearrange:
cos x (tan^2 x +1 )
Apply Pythagorean identity:
cos x * sec ^2 x
Rewrite Sec x in terms of sin and cos
cos x * (1 / cos x )^2
Apply product rule:
cos x * 1^2 / cos^2 x
cos x * (1/ cos^2 x )
Cancel common factor cos x
1 / cos x
1/cosx = sec x
Answer : sec x (option c)
