Math Problem Statement

Find the maximum area of a triangle formed in the first quadrant by the x-axis, 7-axis and a tangent line to the graph of y=(x+7)^-2

Solution

To find the maximum area of a triangle formed by the x-axis, the y-axis, and a tangent line to the graph of y=1(x+7)2y = \frac{1}{(x+7)^2}, we can follow these steps:

Step 1: Find the equation of the tangent line

Given the function y=1(x+7)2y = \frac{1}{(x+7)^2}, let's first find its derivative to determine the slope of the tangent line at any point (x0,y0)(x_0, y_0).

The derivative yy' of the function is:

y=ddx(1(x+7)2)=2(x+7)3=2(x+7)3y' = \frac{d}{dx} \left( \frac{1}{(x+7)^2} \right) = -2(x+7)^{-3} = -\frac{2}{(x+7)^3}

So, the slope of the tangent line at any point (x0,y0)(x_0, y_0) is:

m=2(x0+7)3m = -\frac{2}{(x_0+7)^3}

The equation of the tangent line at this point can be written as:

yy0=m(xx0)y - y_0 = m(x - x_0)

Substituting y0=1(x0+7)2y_0 = \frac{1}{(x_0+7)^2} and m=2(x0+7)3m = -\frac{2}{(x_0+7)^3}, we have:

y1(x0+7)2=2(x0+7)3(xx0)y - \frac{1}{(x_0+7)^2} = -\frac{2}{(x_0+7)^3}(x - x_0)

This simplifies to:

y=2(x0+7)3x+2x0+7(x0+7)3y = -\frac{2}{(x_0+7)^3}x + \frac{2x_0 + 7}{(x_0+7)^3}

Step 2: Find the x- and y-intercepts of the tangent line

The x-intercept occurs when y=0y = 0:

0=2(x0+7)3x+2x0+7(x0+7)30 = -\frac{2}{(x_0+7)^3}x + \frac{2x_0 + 7}{(x_0+7)^3}

x=x0+7x = x_0 + 7

The y-intercept occurs when x=0x = 0:

y=2(x0+7)3(0)+2x0+7(x0+7)3=2x0+7(x0+7)3y = -\frac{2}{(x_0+7)^3}(0) + \frac{2x_0 + 7}{(x_0+7)^3} = \frac{2x_0 + 7}{(x_0+7)^3}

Thus, the x-intercept is x0+7x_0 + 7, and the y-intercept is 2x0+7(x0+7)3\frac{2x_0 + 7}{(x_0+7)^3}.

Step 3: Find the area of the triangle

The area AA of the triangle formed by the x-axis, the y-axis, and the tangent line is given by:

A=12×x-intercept×y-interceptA = \frac{1}{2} \times \text{x-intercept} \times \text{y-intercept}

A=12×(x0+7)×2x0+7(x0+7)3A = \frac{1}{2} \times (x_0 + 7) \times \frac{2x_0 + 7}{(x_0+7)^3}

This simplifies to:

A=(2x0+7)2(x0+7)2A = \frac{(2x_0 + 7)}{2(x_0+7)^2}

Step 4: Maximize the area

To find the value of x0x_0 that maximizes the area, differentiate AA with respect to x0x_0 and set it equal to zero:

dAdx0=ddx0(2x0+72(x0+7)2)\frac{dA}{dx_0} = \frac{d}{dx_0} \left( \frac{2x_0 + 7}{2(x_0+7)^2} \right)

Using the quotient rule, we have:

dAdx0=(2(x0+7)2)(2x0+7)(2(x0+7))2(x0+7)4\frac{dA}{dx_0} = \frac{(2(x_0+7)^2) - (2x_0 + 7)(2(x_0+7))}{2(x_0+7)^4}

Simplify the expression:

dAdx0=2(x0+7)2(2x0+7)(x0+7)3=2(x0+72x07)(x0+7)3=2x0(x0+7)3\frac{dA}{dx_0} = \frac{2(x_0 + 7) - 2(2x_0 + 7)}{(x_0+7)^3} = \frac{2(x_0 + 7 - 2x_0 - 7)}{(x_0 + 7)^3} = \frac{-2x_0}{(x_0 + 7)^3}

Setting dAdx0=0\frac{dA}{dx_0} = 0, we have:

2x0=0-2x_0 = 0

Thus, x0=0x_0 = 0.

Step 5: Calculate the maximum area

Substitute x0=0x_0 = 0 back into the expression for AA:

Amax=2(0)+72(0+7)2=72×49=798=114A_{\text{max}} = \frac{2(0) + 7}{2(0+7)^2} = \frac{7}{2 \times 49} = \frac{7}{98} = \frac{1}{14}

So, the maximum area of the triangle is 114\frac{1}{14}.

Would you like further details, or have any questions?


Here are some related questions for you:

  1. What is the general method to find the tangent to any curve?
  2. How does the area of the triangle change if we change the function y=1(x+7)2y = \frac{1}{(x+7)^2}?
  3. Can this problem be solved using Lagrange multipliers?
  4. How does the area change if the tangent line was in a different quadrant?
  5. What are the geometric interpretations of maxima and minima in calculus?
  6. Can you generalize the result to other curves, e.g., y=1(x+c)2y = \frac{1}{(x+c)^2}?
  7. What if we had different axes, say x=ax = a and y=by = b, instead of the x- and y-axes?
  8. How would the solution change if the function had a different power, e.g., y=1(x+7)3y = \frac{1}{(x+7)^3}?

Tip: When dealing with optimization problems, always check if the function has any boundaries, as this can sometimes simplify finding the maximum or minimum values.

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Math Problem Analysis

Mathematical Concepts

Calculus
Derivatives
Optimization

Formulas

Derivative of a function
Area of a triangle

Theorems

First derivative test for optimization

Suitable Grade Level

Advanced High School or College