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EXAMPLE 1<br />

Solving a tangent problem involving a logarithmic function<br />

Determine the equation of the tangent to y log 2 x at 18, 32.<br />

Solution<br />

dy<br />

The slope of the tangent is given by the derivative , where y log 2 x.<br />

dx<br />

dy<br />

dx 1<br />

x ln 2<br />

At x 8, dy<br />

dx 1<br />

8 ln 2 .<br />

The equation of the tangent is<br />

y 3 1 1x 82<br />

8 ln 2<br />

y 1<br />

8 ln 2 x 3 1<br />

ln 2<br />

We can determine the derivatives of other logarithmic functions using the rule<br />

d<br />

, along with other derivative rules.<br />

dx 1log ax2 1<br />

x ln a<br />

EXAMPLE 2<br />

Selecting a strategy to differentiate a composite logarithmic function<br />

Determine the derivative of y log 4 12x 32 5 .<br />

Solution<br />

We can rewrite the logarithmic function as follows:<br />

y log 4 12x 32 5<br />

y 5 log 4 12x 32<br />

dy<br />

dx d dx 35 log 412x 324<br />

(Property of logarithms)<br />

5 d dx 3log 412x 324<br />

5 d3log 412x 324<br />

d312x 324<br />

1<br />

5 a<br />

12x 32 ln 4 b122<br />

<br />

10<br />

12x 32 ln 4<br />

d12x 32<br />

dx<br />

(Chain rule)<br />

(Simplify)<br />

NEL<br />

CALCULUS APPENDIX—THE DERIVATIVES OF GENERAL LOGARITHMIC FUNCTIONS 577

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