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What is the sum-to-product formula for the sine function?

The sum-to-product formula for the difference of two sine functions is sin⁡A−sin⁡B=2sin⁡A−B2cos⁡A+B2\sin A - \sin B = 2\sin\frac{A-B}{2}\cos\frac{A+B}{2}. It converts a difference of sines into a product of a sine and a cosine function.

Conditions

  • AA and BB are arbitrary real numbers.
  • Angles are in radians for calculus applications.

Reasoning, step by step

  1. Identify the two angles AA and BB.
  2. Calculate the half-difference A−B2\frac{A-B}{2} and the half-sum A+B2\frac{A+B}{2}.
  3. Multiply 2 by the sine of the half-difference and the cosine of the half-sum.

Example

In the video, the formula is applied with A=x+hA = x+h and B=xB = x to transform the numerator of the derivative limit: sin⁡(x+h)−sin⁡x=2sin⁡h2cos⁡2x+h2\sin(x+h) - \sin x = 2\sin\frac{h}{2}\cos\frac{2x+h}{2}.

Common misconceptions

  • Confusing the sum-to-product formula with the product-to-sum formula.
  • Incorrectly calculating the half-sum or half-difference arguments.

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Answers are generated from source material and independently checked. Consult the original video or creator if something is unclear.