Question 111

Mathematics Tangents and Normals Hard

The equation of the tangent at any point of curve \(x=a cos2t, y=2\sqrt{2} a sint\) with \(m\) as its slope is

(A) \(y=mx+a(m-\frac{1}{m})\)
(B) \(y=mx-a(m+\frac{1}{m})\)
(C) \(y=mx+a(a+\frac{1}{a})\)
(D) \(y=amx+a(m-\frac{1}{m})\)
✓ Correct Answer: Option B Verified Solution

📘 Step-by-Step Solution & Detailed Explanation:

\(\frac{dx}{dt} = -2a\sin 2t = -4a\sin t\cos t\) and \(\frac{dy}{dt} = 2\sqrt{2}a\cos t\).

\(m = \frac{dy}{dx} = \frac{2\sqrt{2}a\cos t}{-4a\sin t\cos t} = -\frac{1}{\sqrt{2}\sin t}\), so \(\sin t = -\frac{1}{\sqrt{2}m}\).

The point: \(y_0 = 2\sqrt{2}a\sin t = -\frac{2a}{m}\) and \(x_0 = a(1 - 2\sin^2 t) = a\left(1 - \frac{1}{m^2}\right)\).

Tangent: \(y = mx + (y_0 - mx_0) = mx - \frac{2a}{m} - am + \frac{a}{m} = mx - a\left(m + \frac{1}{m}\right)\)

Answer: (B)

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