Sign In
The sign of each formula is determined by the quadrant where the angle 2θ lies.
These identities are useful when finding the exact value of the sine, cosine, or tangent at a given angle.
LHS−1=RHS−1
LHS/(-2)=RHS/(-2)
Rearrange equation
Put minus sign in front of fraction
-(b−a)=a−b
LHS=RHS
LHS+1=RHS+1
LHS/2=RHS/2
Rearrange equation
LHS=RHS
sin2θ=±21−cosθ, cos2θ=±21+cosθ
ba=ba
c/da/b=ba⋅cd
Cross out common factors
Cancel out common factors
Multiply fractions
Consider the calculation of the exact value of cos15∘.