Mental Calculation of n^2 Base 10

n^2 = (n+(n-10))x10+(n-10)^2

From

n^2 = (n-10)^2 + 20n - 10^2 <<< (n^2 - 20n + 10^2) + 20n - 10^2

example

15^2 = ((15 + 5) x 10 = 200) + ( 5 x 5 = 25) = 225

17^2 = ((17 + 7) x 10 = 240) + ( 7 x 7 = 49) = 289

11^2 = ((11 + 1) x 10 = 120) + (1 x 1 = 1) = 121

9^2 = (( 9 - 1) x 10 = 80) + (-1 x -1 = 1) = 81