Question

using half angle formula solve cos75

Answer 1

plz help

Answer 2

So we might want to try rewriting cos(75) in terms of cos(150), which is 75*2. That is a value on a unit circle.
Just start by writing out the formula for half-angle.

Answer 3

cos(t/2) = +- sqrt( (1 + cos(t)) / 2 )
Where t = 150.

And note that 75 is in the range where cosine is positive, so its value must also be positive. You may eliminate the negative end.