I rewrote the statement as $$ \sin(30^° + 15^°) + \sin(15^°) = \cos(15^°). $$ Then I got $$ (\sqrt{3}-2) \sin(15^°) = \cos(15^°). $$
Asked
Active
Viewed 4,853 times
3
-
Always specify degrees or radians in the argument. Note the answer by TZakrevskiy is specific with degrees. – P T Apr 22 '15 at 18:55
2 Answers
10
$$\sin 75^° - \sin 15^° = 2\sin\left(\frac{75^°-15^°}{2}\right)\cos\left(\frac{75^°+15^°}{2}\right) = 2 \sin 30^° \cos 45^° $$$$= \cos 45 ^°=\sin 45^°.$$
GNUSupporter 8964民主女神 地下教會
- 17,627
TZakrevskiy
- 22,980
1
\begin{align} \sin(45^{°}) + \sin(15^{°}) & = 2 \sin\left(\dfrac{45^{°}+15^{°}}2\right) \cos\left(\dfrac{45^{°}-15^{°}}2\right) = 2 \sin(30^{°})\cos(15^{°})\\ & = 2 \cdot \dfrac12 \cdot \sin(90^{°}-15^{°}) = \sin(75^{°}) \end{align}
GNUSupporter 8964民主女神 地下教會
- 17,627
Adhvaitha
- 20,259
-
@user233631, http://mathworld.wolfram.com/ProsthaphaeresisFormulas.html – lab bhattacharjee Apr 26 '15 at 04:30