The radius of Earth is about 3,980 miles. What expression can be used to find the approximate diameter of Earth?

To find the diameter, you use the formula: 2x. we use 2x because the radius is 2 times the diameter.

so, if the radius is 3,980, then we substitute that into the formula.

2 (3,980)

this will give us 7960. the diameter is 7960 miles.