Use [tex]\sec\theta = \dfrac{1}{\cos{\theta}}[/tex] to get
[tex]\sec\theta -\cos{\theta}= \dfrac{1}{\cos{\theta}}-\cos(\theta})[/tex]
Then find a common denominator:
[tex]\dfrac{1}{\cos{\theta}}-\cos(\theta}) = \dfrac{1}{\cos{\theta}}-\dfrac{\cos^2\theta}{\cos\theta}[/tex]
But now make that one fraction:
[tex]\dfrac{1}{\cos{\theta}}-\dfrac{\cos^2\theta}{\cos\theta} = \dfrac{1 - \cos^2\theta}{\cos{\theta}}[/tex]
But that's just sin^2 on the top
[tex]\dfrac{1 - \cos^2\theta}{\cos{\theta}} = \dfrac{sin^2\theta}{\cos{\theta}} = \dfrac{sin\theta\cdot sin\theta}{\cos{\theta}}[/tex]
Split that fraction up:
[tex]=\dfrac{ sin\theta}{\cos{\theta}}\cdot sin\theta[/tex]
And tan = sin/cos
[tex]=\tan\theta\cdot sin\theta[/tex]
