Math Problem Statement

Consider the curve y = ln(3x - 1). Let P be the point on the curve where x = 2. (a) Write down the gradient of the curve at P. (b) Find the equation of the tangent to the curve at P. (c) The normal to the curve at P cuts the x-axis at R. Find the coordinates of R.

Solution

Let's solve the questions step by step.

The given curve is:

y=ln(3x1)y = \ln(3x - 1) and point PP is where x=2x = 2.


(a) Gradient of the curve at PP

To find the gradient of the curve at PP, we need to differentiate yy with respect to xx and then evaluate it at x=2x = 2.

  1. Differentiate y=ln(3x1)y = \ln(3x - 1):

Using the chain rule: dydx=13x1ddx(3x1)=13x13=33x1\frac{dy}{dx} = \frac{1}{3x - 1} \cdot \frac{d}{dx}(3x - 1) = \frac{1}{3x - 1} \cdot 3 = \frac{3}{3x - 1}

  1. Evaluate at x=2x = 2: dydxx=2=33(2)1=361=35\frac{dy}{dx} \Big|_{x=2} = \frac{3}{3(2) - 1} = \frac{3}{6 - 1} = \frac{3}{5}

Thus, the gradient at PP is: 35\boxed{\frac{3}{5}}


(b) Equation of the tangent to the curve at PP

The equation of the tangent line is given by the point-slope form: yy1=m(xx1)y - y_1 = m(x - x_1) where mm is the gradient, and (x1,y1)(x_1, y_1) is the point of tangency.

  1. From part (a), the gradient m=35m = \frac{3}{5}.

  2. The point PP is where x=2x = 2. Let's find yy at x=2x = 2: y=ln(3(2)1)=ln(61)=ln(5)y = \ln(3(2) - 1) = \ln(6 - 1) = \ln(5) So, the coordinates of PP are (2,ln(5))(2, \ln(5)).

  3. Substitute into the equation: yln(5)=35(x2)y - \ln(5) = \frac{3}{5}(x - 2) Simplify: y=35(x2)+ln(5)y = \frac{3}{5}(x - 2) + \ln(5)

Thus, the equation of the tangent is: y=35x65+ln(5)\boxed{y = \frac{3}{5}x - \frac{6}{5} + \ln(5)}


(c) Coordinates of RR where the normal to the curve at PP cuts the xx-axis

  1. The gradient of the normal is the negative reciprocal of the gradient of the tangent. So, the gradient of the normal is: mnormal=53m_{\text{normal}} = -\frac{5}{3}

  2. The equation of the normal is also given by the point-slope form: yln(5)=53(x2)y - \ln(5) = -\frac{5}{3}(x - 2) Simplify: y=53(x2)+ln(5)y = -\frac{5}{3}(x - 2) + \ln(5) y=53x+103+ln(5)y = -\frac{5}{3}x + \frac{10}{3} + \ln(5)

  3. To find the x-intercept, set y=0y = 0: 0=53x+103+ln(5)0 = -\frac{5}{3}x + \frac{10}{3} + \ln(5) Multiply through by 3 to eliminate the fractions: 0=5x+10+3ln(5)0 = -5x + 10 + 3\ln(5) Solve for xx: 5x=10+3ln(5)5x = 10 + 3\ln(5) x=2+3ln(5)5x = 2 + \frac{3\ln(5)}{5}

Thus, the x-coordinate of RR is: x=2+3ln(5)5\boxed{x = 2 + \frac{3\ln(5)}{5}}

The coordinates of RR are (2+3ln(5)5,0)\left(2 + \frac{3\ln(5)}{5}, 0\right).


Do you want further details or have any questions?
Here are some related questions to expand on this:

  1. How do you find the second derivative of y=ln(3x1)y = \ln(3x - 1)?
  2. Can you find the point where the curve has zero gradient?
  3. What is the area under the curve between x=1x = 1 and x=2x = 2?
  4. How would the equation of the tangent change if the curve was y=ln(2x3)y = \ln(2x - 3)?
  5. What is the significance of the normal line's x-intercept in this problem?

Tip: When dealing with logarithmic functions, remember that their derivatives follow the chain rule and are often simplified using properties of logarithms.

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Math Problem Analysis

Mathematical Concepts

Differentiation
Tangent and Normal lines
Logarithmic Functions

Formulas

Derivative of ln(3x - 1): d/dx[ln(3x - 1)] = 3/(3x - 1)
Point-slope form for tangent: y - y1 = m(x - x1)
Gradient of normal: -1/m

Theorems

Chain Rule
Point-slope form for linear equations

Suitable Grade Level

Grade 11-12 (Advanced Calculus)