Geometry
# Sum and Difference Trigonometric Formulas

If $\sin\theta= \cos\theta$, find the value of $\cos 2\theta$.

If $A + B = 225^{\circ}$, find $(1+\tan A)(1+\tan B),$ where defined.

$\large \frac {1}{\cos 80^\circ } - \frac {\sqrt 3}{\sin 80^\circ } = \ ?$

**Hint:** How can we write the numerators in relation to common trig values?

$\sin^2 18^ \circ + \sin ^2 30 ^ \circ.$

Which of the following is equal to the above expression?