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?

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