Find the exact value of sin²75°, given that sin15° = (√3 -1)/2√2.

Could someone please explain to me the relevance of sin15° in finding sin²75°?

Thanx

Printable View

- Oct 7th 2008, 12:11 AMxwrathbringerxExact Value - Please Help?!
Find the exact value of sin²75°, given that sin15° = (√3 -1)/2√2.

Could someone please explain to me the relevance of sin15° in finding sin²75°?

Thanx - Oct 7th 2008, 12:18 AMGreengoblin
presumably you are expected to know that $\displaystyle sin(60)=\frac{\sqrt{3}}{2}$ and $\displaystyle sin(\theta+\phi)=sin(\theta)cos(\phi)+cos(\theta)s in(\phi)$

- Oct 7th 2008, 12:20 AMxwrathbringerx
errr sori but that's made me more confused (Worried)

- Oct 7th 2008, 01:08 AMGreengoblin
Sorry, whats confusing you? often you are expected to memorize the sin and cosine of some comon angles, such as $\displaystyle 30^{\circ},45^{\circ},60^{\circ}, $etc.

The fomula is for the sine of the sum of two angles. Have you seen this yet? I just realised you don't have the cosines of the angles anyway, so probably you have to draw a diagram, label the sides of the triangles, and remember $\displaystyle cos(\theta)=\frac{adjacent}{hypotenuse}$ - Oct 7th 2008, 01:13 AMxwrathbringerx
oh i get it ...how stupid of me

thanx