The parametric equations of a curve are
\(x = 2\theta + \sin 2\theta, \quad y = 1 - \cos 2\theta.\)
Show that \(\frac{dy}{dx} = \tan \theta.\)
Solution
First, find \(\frac{dx}{d\theta}\) and \(\frac{dy}{d\theta}\):
\(\frac{dx}{d\theta} = 2 + 2\cos 2\theta\)
\(\frac{dy}{d\theta} = 2\sin 2\theta\)
Now, use the chain rule to find \(\frac{dy}{dx}\):
\(\frac{dy}{dx} = \frac{dy}{d\theta} \div \frac{dx}{d\theta} = \frac{2\sin 2\theta}{2 + 2\cos 2\theta}\)
Simplify the expression:
\(\frac{dy}{dx} = \frac{2\sin 2\theta}{2(1 + \cos 2\theta)} = \frac{\sin 2\theta}{1 + \cos 2\theta}\)
Using the identity \(\sin 2\theta = 2\sin \theta \cos \theta\) and \(1 + \cos 2\theta = 2\cos^2 \theta\), we have:
\(\frac{dy}{dx} = \frac{2\sin \theta \cos \theta}{2\cos^2 \theta} = \frac{\sin \theta}{\cos \theta} = \tan \theta\)
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