• Oct 7th 2008, 12:11 AM
xwrathbringerx
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 AM
Greengoblin
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 AM
xwrathbringerx
errr sori but that's made me more confused (Worried)
• Oct 7th 2008, 01:08 AM
Greengoblin
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 AM
xwrathbringerx
oh i get it ...how stupid of me

thanx