Practicing Success
If cos 50° + sin 50° = K, then what is the value of cos3 50° - sin3 50°? |
\(\frac{K^2 - 1}{2}\) \(\sqrt {2 - K^2}\) \(\frac{K^2 + 1}{2}\) \(\sqrt {2 - K^2}\) \(\frac{K^2 - 1}{2}\) \(\sqrt {2 + K^2}\) \(\frac{K^2 + 1}{2}\) \(\sqrt {2 + K^2}\) |
\(\frac{K^2 + 1}{2}\) \(\sqrt {2 - K^2}\) |
⇒ cos 50° + sin 50° = K ......(i) So, cos 50° - sin 50° = \(\sqrt {2 - K^2}\) on squaring equation (i) ⇒ 1 + 2cos 50° sin 50° = K2 ⇒ cos 50° sin 50° = \(\frac{K^2 - 1}{2}\) Now, cos3 50° - sin3 50° = (cos 50° - sin 50°) (cos2 50° + sin2 50° + cos 50° sin 50°) = \(\sqrt {2 - K^2}\) × [ 1 + \(\frac{K^2 - 1}{2}\)] = \(\frac{(K^2 + 1)}{2}\) \(\sqrt {2 - K^2}\) |