June 2021 p11 q11
1176
The equation of a curve is \(y = 2\sqrt{3x+4} - x\).
Find the equation of the normal to the curve at the point (4, 4), giving your answer in the form \(y = mx + c\).
Solution
First, find the derivative \(\frac{dy}{dx}\) of the curve \(y = 2\sqrt{3x+4} - x\).
Using the chain rule, \(\frac{dy}{dx} = 3(3x+4)^{-0.5} - 1\).
Substitute \(x = 4\) into \(\frac{dy}{dx}\) to find the gradient of the tangent at the point (4, 4):
\(\frac{dy}{dx} = \frac{1}{4}\).
The gradient of the normal is the negative reciprocal of the gradient of the tangent:
\(m = -\frac{1}{\frac{1}{4}} = 4\).
Using the point-slope form of a line, \(y - y_1 = m(x - x_1)\), with \(m = 4\) and point (4, 4):
\(y - 4 = 4(x - 4)\).
Simplify to find the equation of the normal:
\(y = 4x - 16 + 4\).
Thus, \(y = 4x - 12\).
Log in to record attempts.