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Calculus- Early Transcendentals, 2021a

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2.5. Logarithms 61<br />

We also give it special notation:<br />

log e x = lnx.<br />

You may pronounce ln as either: “el - en”, “lawn”, or refer to it as “natural log”. The above properties<br />

of logarithms also apply to the natural logarithm.<br />

Often we need to turn a logarithm (in a different base) into a natural logarithm. This gives rise to the<br />

change of base formula.<br />

Change of Base Formula<br />

log a x = lnx<br />

lna .<br />

Example 2.19: Combine Logarithms<br />

Write lnA + 2lnB − lnC as a single logarithm.<br />

Solution. Using properties of logarithms, we have,<br />

lnA + 2lnB − lnC = lnA + lnB 2 − lnC<br />

= ln(AB 2 ) − lnC<br />

= ln AB2<br />

C<br />

♣<br />

Example 2.20: Solve Exponential Equations using Logarithms<br />

If e x+2 = 6e 2x , then solve for x.<br />

Solution. Taking the natural logarithm of both sides and noting the cancellation formulas (along with<br />

lne = 1), we have:<br />

e x+2 = 6e 2x<br />

lne x+2 = ln(6e 2x )<br />

x + 2 = ln6 + lne 2x<br />

x + 2 = ln6 + 2x<br />

x = 2 − ln6<br />

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