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Logarithms

Rules

Since the notion of a logarithm is derived from exponents, all logarithmic rules for multiplication, division and raised to a power are based on those for exponents. There are no general rules for the logarithms of sums and differences.

Recall that when we

  • multiply two powers we add their exponents
    bmbn = bm+n
  • divide one power by another we subtract the exponents
    b^m/b^n = bm−n
  • raise one power by a number we multiply the exponent by that number.
    (bm)n = bmn

Translating these rules to logarithms we obtain:

Rules

For all positive real numbers,

logb(xy) = logb x + logb y

log base b of x/y is equal to log base b of x minus log base b of y

logb(xn) = n logb x

with Special Cases:

logb b = 1, as b1 = b

logb 1= 0, as b0 = 1

Example

Express the logarithm of a number in terms of the logarithms of as few numbers of primes as possible. Try some of these yourself and check against the given answers. There are 8 examples to view.



log10

 

decomposition of x

log10
( )

 

log10x written in terms of logs of primes

 

log10x written as a decimal

(2 d.p.)

 

 


Example

Express the logarithm of an expression in terms of the logarithms of as few numbers of primes as possible. Try some of these exercises yourself and check against the given answers. There are 8 examples given.



log10

 

decomposition of x

log10
( )

 

log10x written in terms of logs of primes

 

log10x written in decimal form

(2 d.p.)

 

 

 

<< The General Logarithm (Definition) | Logarithms Index | The Common Logarithm >>

 

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