If 20 × 20 = 400, execute the difference of two squares to find 19×21?
I have no idea how to do this. any help would be great please?
The difference of two squares is $\displaystyle x^2-y^2=(x-y)(x+y)$. Let $\displaystyle x=20$ and $\displaystyle y=1$. Do you see where to go from here?