# Logarithm property

An important area of application for base 10 logarithms is when you want to solve equations containing *x* as an exponent.

**Example**

$$6^{x}=20$$

Now that we know that a number may be rewritten as an exponent of 10, we can start by rewriting 6 and 20:

$$6=10^{\log 6}$$

$$20=10^{\log 20}$$

We can insert these notations in an equation again:

$$(10^{\log 6})^{x}=10^{\log 20}$$

A rule from a earlier chapter:

$$(a^{b})^{c}=a^{bc}$$

Provides:

$$10^{x\log 6}=10^{\log 20}$$

Now we can see that both exponents equal each other and the left-hand side = right-hand side:

$$x\cdot \log 6=\log 20$$

Here we need only isolate *x*:

$$x=\frac{log20}{log6}\approx 1.67$$

There are some properties of logarithms that are important to master:

### First the product property

$$log_{b}(ac)=log_{b}(a)+log_{b}(c)$$

$$a, b,c\; positive\; numbers,\; b\neq 1$$

### Second the quotient property

$$log_{b}\left(\frac{a}{c}\right)=log_{b}(a)-log_{b}(c)$$

$$a, b,c\; positive\; numbers,\; b\neq 1$$

### Third the power property

$$ log_{b}\left(a^{c}\right)=c\cdot log_{b}(a)$$

$$c=real\; number,a\; and\; b\; positive\; numbers,\; b\neq 1$$

**Video lesson**

We wish to invest $ 1 000 in a fund that pays 12% in interest. How many years would it take for this investment to grow until we have $ 10 000 in the fund?