## Trigonometry 7th Edition

To find exact value of $\tan 135^{\circ}$, let's find its reference angle first. As $135^{\circ}$ terminates in quadrant II, The reference angle = $180^{\circ} - 135^{\circ}$ = $45^{\circ}$ As $135^{\circ}$ terminates in quadrant II, its $\tan$ will be negative. Therefore by reference angle theorem- $\tan 135^{\circ}$ = - $\tan45^{\circ}$ = - 1