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Which of the following is equal to sin(π5) \sin \left( \frac{\pi}{5} \right) ?\newlineA) cos(π5) -\cos \left( \frac{\pi}{5} \right) \newlineB) sin(π5) -\sin \left( \frac{\pi}{5} \right) \newlineC) cos(3π10) \cos \left( \frac{3\pi}{10} \right) \newlineD) sin(7π10) \sin\left( \frac{7\pi}{10} \right)

Full solution

Q. Which of the following is equal to sin(π5) \sin \left( \frac{\pi}{5} \right) ?\newlineA) cos(π5) -\cos \left( \frac{\pi}{5} \right) \newlineB) sin(π5) -\sin \left( \frac{\pi}{5} \right) \newlineC) cos(3π10) \cos \left( \frac{3\pi}{10} \right) \newlineD) sin(7π10) \sin\left( \frac{7\pi}{10} \right)
  1. Understand Trigonometric Identities: Step 11: Understand the trigonometric identities. sin(π5)\sin(\frac{\pi}{5}) is a positive value since π5\frac{\pi}{5} is in the first quadrant.
  2. Evaluate Option A: Step 22: Evaluate option A.\newlineA) cos(π5)−\cos(\frac{\pi}{5}) - Since cos(π5)\cos(\frac{\pi}{5}) is positive in the first quadrant, cos(π5)−\cos(\frac{\pi}{5}) is negative. This cannot be equal to sin(π5)\sin(\frac{\pi}{5}) which is positive.
  3. Evaluate Option B: Step 33: Evaluate option B.\newlineB) sin(π5)-\sin(\frac{\pi}{5}) - This is simply the negative of sin(π5)\sin(\frac{\pi}{5}), which is positive. So, this option is also incorrect.
  4. Evaluate Option C: Step 44: Evaluate option C.\newlineC) cos(3π10)\cos(\frac{3\pi}{10}) - Using the identity cos(θ)=sin(π2θ)\cos(\theta) = \sin(\frac{\pi}{2} - \theta), we get cos(3π10)=sin(π23π10)=sin(π5)\cos(\frac{3\pi}{10}) = \sin(\frac{\pi}{2} - \frac{3\pi}{10}) = \sin(\frac{\pi}{5}). This matches sin(π5)\sin(\frac{\pi}{5}).
  5. Evaluate Option D: Step 55: Evaluate option D.\newlineD) sin(7π10)\sin(\frac{7\pi}{10}) - Using the identity sin(πθ)=sin(θ)\sin(\pi - \theta) = \sin(\theta), we get sin(7π10)=sin(π7π10)=sin(3π10)\sin(\frac{7\pi}{10}) = \sin(\pi - \frac{7\pi}{10}) = \sin(\frac{3\pi}{10}). This is not equal to sin(π5)\sin(\frac{\pi}{5}).

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