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**Prove It** Sine is positive in the first and second quadrants. An angle of $\displaystyle \displaystyle \frac{3\pi}{4}$ is the same as an angle of $\displaystyle \displaystyle \frac{\pi}{4}$ in the second quadrant. $\displaystyle \displaystyle \frac{\pi}{4} = 45^{\circ}$.

Draw a $\displaystyle \displaystyle 45^{\circ}, 45^{\circ}, 90^{\circ}$ right-angle triangle of side lengths $\displaystyle \displaystyle 1, 1, \sqrt{2}$. You should be able to see why $\displaystyle \displaystyle \sin{45^{\circ}} = \frac{1}{\sqrt{2}}$.