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Double Angle Formula

The Double Angle Formulas

Also known as double angle identities, there are three distinct double angle formulas: sine, cosine, and tangent. The cosine double angle formula has three variations. The double angle formulas relate different trigonometry functions to each other.

The three double angle formulas and their variations are given as:
sin(2θ) = 2sin(θ)cos(θ)
cos(2θ) = cos2(θ) – sin2(θ)
cos(2θ) = 2cos2(θ) – 1
cos(2θ) = 1 – 2sin2(θ)
tan(2θ) = [2tan(θ)]/[1 – tan2(θ)]

When to use the Formulas

Each double angle formula is useful for simplifying expressions that contain trigonometric terms. For example, an expression may have the term cos(2θ) in it. We can substitute the term 1 – 2sin2(θ) in for cos(2θ). This will help us if we need the expression to be in terms in sine.

Here’s another instance of using a double angle formula to simplify an expression. Imagine coming across the expression sin2(θ) – cos2(θ). Not too bad, but having to hand calculate the value of this expression by plugging in angles could be difficult. The first cosine formula tells us that we can replace sin2(θ) – cos2(θ) with -cos(2θ). It is now is much simpler and more manageable to hand calculate.

Example Problem

Prove the sine double angle formula by using the angle θ = 4 radians.

sin(4) = -0.757
Let’s plug in our values to the double angle formula.
sin(2·4) = 2sin(4)cos(4)
0.989 = 2(-0.757)(-0.654)
0.989 = 0.989 √

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