A circle has a circumference of 31.41.
What is its radius?
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A circle has a circumference of 31.41.
What is its radius?
To solve the exercise, first we must remember the circumference formula:
P is the circumference and Pi has a value of 3.14 (approximately).
We substitute in the known data:
Keep in mind that the result can be easily simplified using Pi:
Finally, we simplify by 2:
5
Look at the circle in the figure below.
The diameter of the circle is 4.
What is its perimeter?
The circumference formula is C = 2πr, not C = πr. To isolate r, you must divide by the entire coefficient 2π. Think of it as undoing multiplication!
Use the value that matches your given circumference! Here, 31.41 ÷ 6.282 = 5 exactly, so 3.141 gives the cleanest answer. More decimal places mean more precision.
Radius goes from center to edge (what we're finding). Diameter goes all the way across and equals 2 × radius. Don't confuse them!
A quick check: if radius = 5, then diameter = 10. Does a circle with diameter 10 having circumference 31.41 make sense? Yes, since circumference ≈ 3 × diameter!
That's normal! Real-world problems often have decimal answers. Just round appropriately (usually 1-2 decimal places) and always check your work.
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