# homework help: given the diameter, what is the radius?

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#### Guest

##### Guest
Diameter of a cirlce:
If the diameter of a circle is 1.3 * 10(-12) meters, than what is it's radius?
The 10(-12) is the number 10 with a negative 12 exponent.
Thank you-Caron

#### jonboy

##### Full Member
Hello Caron!

If the diameter of a circle is 1.3 * 10(-12) meters, than what is it's radius?
The 10(-12) is the number 10 with a negative 12 exponent.
Thank you-Caron
Ok I am sure you know that the radius is half the diameter. Whenever using Scientic Notation you do not do $$\displaystyle \frac{10^-12}{2}=10^-6$$ or anything funky like that.

So: $$\displaystyle \L \;\frac{1.3\,\bullet\,10^ -12}{2}=r$$

Divide 1.3 by 2: $$\displaystyle \;.65\,\bullet\,10^-12=r$$

In scientific notation we always want the decimal to be after the second-from-last number going from the left in this case.

Hence: $$\displaystyle \L \;6.5\,\bullet\,10^-13=r$$

Since making .65 -------> 6.5 that makes the decimal move closer, thus making us add another -1 to the exponent since it is now even further from the .65. Negative exponents means move the decimal to the left.

Hope this helps.

#### Denis

##### Senior Member
1.3 * 10^(-12)
= 1.3 / 10^12
= 13 / 10^13
Now dividing by 2:
= 6.5 / 10^13
= 65 / 10^14 : that's .000 000 000 000 65; quite a small circle :shock: