12. \( \cos \theta\left(1+\tan ^{2} \theta\right) \)

Step 1 ：\( \cos \theta\left(1+\tan ^{2} \theta\right) \)

Step 2 ：\( \cos \theta\left(1+\frac{\sin ^{2} \theta}{\cos ^{2} \theta}\right) \)

Step 3 ：\( \cos \theta\left(\frac{\cos ^{2} \theta + \sin ^{2} \theta}{\cos ^{2} \theta}\right) \)

Step 4 ：\( \cos \theta\left(\frac{1}{\cos ^{2} \theta}\right) \)