The radius is 10 because the radius is half of the diameter. Half of 20 is 10. So substitute that in the radius symbol.
A= 3.14(10)²
Square 10, which is 10 times 10. 10 times 10 equals 100.
A= 3.14(100)
100 times 3.14 is 314 because 100 has two zeroes which means you move the decimal point in 3.14 two places to the right. It gets you 314.0 which is just 314.
