What are the dimensions of the Mac Book Air 13" screen?
To calculate the screen dimension given the diagonal measurement and the resolution:

The 13.3" MacBook has a 1400 x 900 resolution. Assuming square pixel this gives us



which we can solve for x



The resulting value of x needs to be multiplied with 1400 and 900 to give the screen dimensions, in this case 11.2" x 7.2".