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EXAMPLE 2<br />

Selecting a strategy to solve a tangent problem<br />

Determine the equation of the line that is tangent to y <br />

where x 1.<br />

ln x2<br />

3x<br />

at the point<br />

Solution<br />

ln 1 0, so y 0 when x 1, and the point of contact of the tangent is (1, 0).<br />

The slope of the tangent is given by<br />

dy<br />

dx .<br />

3x a 1 b 2x 3 ln x2<br />

dy<br />

2<br />

dx x<br />

9x 2<br />

(Quotient rule)<br />

6 3 ln x2<br />

<br />

9x 2<br />

dy<br />

When x 1, dx 2 3 .<br />

The equation of the tangent is<br />

y 0 2 1x 12,<br />

3<br />

or 2x 3y 2 0.<br />

EXAMPLE 3<br />

Determining where the minimum value of a function occurs<br />

a. For the function f 1x2 x ln x, x 7 0, use your graphing calculator to<br />

determine the x-value that minimizes the value of the function.<br />

b. Use calculus methods to determine the exact x-value where the minimum<br />

is attained.<br />

Solution<br />

a. The graph of f 1x2 x ln x is shown.<br />

Use the minimum value operation of your calculator to determine the minimum<br />

value of f 1x2. The minimum value occurs at x 4.<br />

b. f 1x2 x ln x<br />

To minimize f 1x2, set the derivative equal to zero.<br />

f ¿1x2 1<br />

2x 1 x<br />

1<br />

2x 1 x 0<br />

1<br />

2x 1 x<br />

x 2x<br />

x 2 4x<br />

NEL<br />

CALCULUS APPENDIX—THE NATURAL LOGARITHM AND ITS DERIVATIVE 573

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