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Prove that: tan13x – tan 9x – tan 4x = tan 13x tan 9x tan 4x |
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Answer» We have 13x = 9x + 4x ⇒ tan 13x = tan(9x + 4x) We know that tan(A + B) = \(\frac{tanA+tanB}{1-tanAtanB}\) ⇒ tan 13x = \(\frac{tan9x+tan4x}{1-tan9x\,tan4x}\) ⇒ tan 13x(1 – tan 9x tan 4x) = tan 9x + tan 4x ⇒ tan 13x – tan 13x tan 9x tan 4x = tan 9x + tan 4x ∴ tan 13x – tan 9x – tan 4x = tan 13x tan 9x tan 4x Hence proved. |
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