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Simplify the expression: (cos 6B + cos 2B) / (sin 6B + sin 2B)

Mathematics

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Answer

We will simplify the left-hand side of the expression cos⁡6B+cos⁡2Bsin⁡6B+sin⁡2B\frac{\cos 6B + \cos 2B}{\sin 6B + \sin 2B} using sum-to-product trigonometric identities.

Step-by-step solution

5 steps
  1. State the sum-to-product identities.

    The relevant identities are:

    cos⁡X+cos⁡Y=2cos⁡(X+Y2)cos⁡(X−Y2)\cos X + \cos Y = 2 \cos \left(\frac{X+Y}{2}\right) \cos \left(\frac{X-Y}{2}\right) sin⁡X+sin⁡Y=2sin⁡(X+Y2)cos⁡(X−Y2)\sin X + \sin Y = 2 \sin \left(\frac{X+Y}{2}\right) \cos \left(\frac{X-Y}{2}\right)
  2. Apply the identity to the numerator.

    For the numerator, we have cos⁡6B+cos⁡2B\cos 6B + \cos 2B. Let X=6BX = 6B and Y=2BY = 2B.

    cos⁡6B+cos⁡2B=2cos⁡(6B+2B2)cos⁡(6B−2B2)\cos 6B + \cos 2B = 2 \cos \left(\frac{6B+2B}{2}\right) \cos \left(\frac{6B-2B}{2}\right) =2cos⁡(8B2)cos⁡(4B2)= 2 \cos \left(\frac{8B}{2}\right) \cos \left(\frac{4B}{2}\right) =2cos⁡(4B)cos⁡(2B)= 2 \cos(4B) \cos(2B)
  3. Apply the identity to the denominator.

    For the denominator, we have sin⁡6B+sin⁡2B\sin 6B + \sin 2B. Let X=6BX = 6B and Y=2BY = 2B.

    sin⁡6B+sin⁡2B=2sin⁡(6B+2B2)cos⁡(6B−2B2)\sin 6B + \sin 2B = 2 \sin \left(\frac{6B+2B}{2}\right) \cos \left(\frac{6B-2B}{2}\right) =2sin⁡(8B2)cos⁡(4B2)= 2 \sin \left(\frac{8B}{2}\right) \cos \left(\frac{4B}{2}\right) =2sin⁡(4B)cos⁡(2B)= 2 \sin(4B) \cos(2B)
  4. Substitute the simplified numerator and denominator back into the original expression.

    cos⁡6B+cos⁡2Bsin⁡6B+sin⁡2B=2cos⁡(4B)cos⁡(2B)2sin⁡(4B)cos⁡(2B)\frac{\cos 6B + \cos 2B}{\sin 6B + \sin 2B} = \frac{2 \cos(4B) \cos(2B)}{2 \sin(4B) \cos(2B)}

    Cancel out the common terms 2 and cos⁡(2B)\cos(2B) (assuming cos⁡(2B)≠0\cos(2B) \neq 0).

    =cos⁡(4B)sin⁡(4B)= \frac{\cos(4B)}{\sin(4B)}

    Using the identity cos⁡θsin⁡θ=cot⁡θ\frac{\cos \theta}{\sin \theta} = \cot \theta:

    =cot⁡(4B)= \cot(4B)
  5. Compare the result with the right-hand side of the given identity.

    The left-hand side simplifies to cot⁡(4B)\cot(4B). The question asks to show that it equals tan⁡(4B)\tan(4B).
    Since $\cot

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