Math Problem Statement

Determine the equation of the tangent line and the normal line at the point (9, 3/14) for the function y = √x / (x + 5).

Solution

To solve this problem, we need to follow two steps:

Given:

The function is y=xx+5y = \frac{\sqrt{x}}{x + 5}.

We are tasked with determining:

  1. The equation of the tangent line at the point (9,314)\left(9, \frac{3}{14} \right).
  2. The equation of the normal line at the same point.

Step 1: Tangent Line

The equation of the tangent line at a point (x1,y1)(x_1, y_1) is given by: yy1=mt(xx1)y - y_1 = m_t(x - x_1) Where mtm_t is the slope of the tangent line, which is found by differentiating the given function.

Differentiate y=xx+5y = \frac{\sqrt{x}}{x + 5}

We can apply the quotient rule for differentiation:

ddx(xx+5)=(x+5)ddx(x)xddx(x+5)(x+5)2\frac{d}{dx} \left( \frac{\sqrt{x}}{x + 5} \right) = \frac{(x + 5) \cdot \frac{d}{dx}(\sqrt{x}) - \sqrt{x} \cdot \frac{d}{dx}(x+5)}{(x + 5)^2}

  • The derivative of x\sqrt{x} is 12x\frac{1}{2\sqrt{x}}.
  • The derivative of x+5x + 5 is 1.

Thus, the derivative (slope of the tangent line) is: ddx(xx+5)=(x+5)12xx(x+5)2\frac{d}{dx} \left( \frac{\sqrt{x}}{x + 5} \right) = \frac{(x + 5) \cdot \frac{1}{2\sqrt{x}} - \sqrt{x}}{(x + 5)^2}

Evaluate the derivative at x=9x = 9

Substitute x=9x = 9 into the derivative to find the slope of the tangent line mtm_t.

mt=(9+5)1299(9+5)2m_t = \frac{(9 + 5) \cdot \frac{1}{2\sqrt{9}} - \sqrt{9}}{(9 + 5)^2} mt=14163142m_t = \frac{14 \cdot \frac{1}{6} - 3}{14^2} mt=1463196=733196=7393196=23196=2588=1294m_t = \frac{\frac{14}{6} - 3}{196} = \frac{\frac{7}{3} - 3}{196} = \frac{\frac{7}{3} - \frac{9}{3}}{196} = \frac{-\frac{2}{3}}{196} = -\frac{2}{588} = -\frac{1}{294}

Thus, the slope of the tangent line is mt=1294m_t = -\frac{1}{294}.

Now, we use the point-slope form of the tangent line equation with the point (9,314)\left(9, \frac{3}{14}\right):

y314=1294(x9)y - \frac{3}{14} = -\frac{1}{294}(x - 9) Simplify to get the equation of the tangent line.

Step 2: Normal Line

The slope of the normal line is the negative reciprocal of the tangent line slope. Since mt=1294m_t = -\frac{1}{294}, the slope of the normal line mnm_n is:

mn=294m_n = 294

Now use the point-slope form for the normal line:

y314=294(x9)y - \frac{3}{14} = 294(x - 9) Simplify to get the equation of the normal line.

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Math Problem Analysis

Mathematical Concepts

Derivatives
Tangent and Normal Lines
Differentiation
Quotient Rule

Formulas

Equation of a tangent line: y - y1 = mt(x - x1)
Derivative of √x: d/dx(√x) = 1/(2√x)
Quotient Rule: d/dx(f/g) = (g(df/dx) - f(dg/dx)) / g²

Theorems

Quotient Rule
Point-Slope Form

Suitable Grade Level

Grades 11-12 or College Calculus