OK - remember you asked for it.
A 15.6 in laptop.
15.6 = the diagonal (lower corner to upper corner
For a wide screen ratio: 16::9 (Diagonal = sq root of (16 x16 + 9 x 9) = 18.36 In.
Sin angle = 9 / 18.36 = .49
Cosine angle = 16 / 18.36 = .871
OK for any 16::9 wide screen monitor (or laptop screen:
Horizontal length = .871 x Diagonal ie 15.6 x .871 = 13.59 in
Vertical = .49 x 15.6 = 7.64 In.
For a 17.3” laptop 17.3 x .871 = 15.07 in and 17.3 x .49 = 8.48 In.
You can duplicate this for a 16:10 ratio or the old 4::3 ratio