## Trig Identities

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Trig identities are mathematical formulas that relate the values of trigonometric functions. These identities allow us to simplify expressions, perform calculations, and solve equations involving trig functions. Here are some of the most common trig identities, along with explanations and examples:

1. Pythagorean identities:

– sin²θ + cos²θ = 1

– tan²θ + 1 = sec²θ

– 1 + cot²θ = csc²θ

These identities express the fundamental relationship between the three basic trig functions: sine, cosine, and tangent, and relate them to the other three: cosecant, secant and cotangent. For example, to prove the first identity, use the Pythagorean theorem: a² + b² = c², where a = sinθ, b = cosθ , and c = 1

2. Even and odd identities:

– sin(-θ) = -sin(θ)

– cos(-θ) = cos(θ)

– tan(-θ) = -tan(θ)

These identities indicate that the trigonometric functions are either even or odd, depending on the sign of their angles. For example, sin(θ) is an odd function, because sin(-θ)=-sin(θ). On the other hand, cos(θ) is an even function, because cos(-θ)=cos(θ).

3. Angle sum and difference identities:

– sin(α ± β) = sinαcosβ ± cosαsinβ

– cos(α ± β) = cosαcosβ ∓ sinαsinβ

– tan(α ± β) = (tanα ± tanβ) / (1 ∓ tanαtanβ)

These identities allow us to find the values of sine, cosine, and tangent of sums and differences of angles. For example, to find sin(π/4 + π/3), we use the identity sin(α+β) = sinαcosβ + cosαsinβ with α=π/4 and β=π/3.

4. Double angle identities:

– sin2θ = 2sinθcosθ

– cos2θ = cos²θ – sin²θ

– tan2θ = (2tanθ) / (1 – tan²θ)

These identities express the values of sine, cosine, and tangent of twice an angle in terms of the values of sine, cosine, and tangent of the original angle. For example, to find cos(2π/3), we use the identity cos2θ = cos²θ – sin²θ with θ=π/3.

5. Half angle identities:

– sin(θ/2) = ±√[(1-cosθ)/2]

– cos(θ/2) = ±√[(1+cosθ)/2]

– tan(θ/2) = ±√[(1-cosθ)/(1+cosθ)]

These identities express the values of sine, cosine, and tangent of half an angle in terms of the values of sine, cosine, and tangent of the original angle. For example, to find sin(π/8), we use the identity sin(θ/2) = ±√[(1-cosθ)/2] with θ=π/4.

Trig identities are an important tool for solving various problems in mathematics, physics, and engineering. These formulas can be used to simplify complex expressions, transform equations, and calculate unknown values. Therefore, it is essential to know and understand these identities thoroughly to be successful in math and related fields.

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