# Math Help - Circle's Diameter

1. ## Circle's Diameter

If the circumference of a circle is doubled, the diameter of the circle

(1) remains the same
(2) increases by 2
(3) is multiplied by 4
(4) is doubled

2. Originally Posted by symmetry
If the circumference of a circle is doubled, the diameter of the circle

(1) remains the same
(2) increases by 2
(3) is multiplied by 4
(4) is doubled
Is doubled.

$C=\pi d$
Thus,
$2C=\pi (2d)$

3. ## ok

If the circumference of a circle is 12, the the diameter = pi(12)(d), right?

What whatever the circumference equals, the diameter increases by the same amount, right?

4. Originally Posted by symmetry
If the circumference of a circle is 12, the the diameter = pi(12)(d), right?

What whatever the circumference equals, the diameter increases by the same amount, right?
Right

5. Originally Posted by symmetry
If the circumference of a circle is 12, the the diameter = pi(12)(d), right?...
Hello,

you know already that the circumference of a circle is calculated by:

$c=2 \cdot \pi \cdot r= \pi \cdot d$, with diameter = 2 * radius.

Plug in the value you know:

$12= \pi \cdot d \Longleftrightarrow d=\frac{12}{\pi}$.

In general:

$c=\pi \cdot d \Longrightarrow d=\frac{12}{\pi} \Longrightarrow \pi=\frac{c}{d}$

EB

PS: I've attached a diagram which helps to memorize the right formula: Cover the variable you want to calculate with your fingertip. The remaining term shows how to calculate the covered value.

6. ## ok

Thank you for the notes and reply.

Earboth,

Thank you for the picture. I think tutors should use pictures here to help students.