A = L x W equation 1 let D = diagonal
or 192 = LW
D = L^2 + W^2 equation 2
or 20 = L^2 + W^2
let W in terms in L in equation 2
and it comes up to this
W= sqroot(20 + L^2)
then
substitute the W from the equation 2 to equation 1
and the equation 1 becomes
192= L * sqroot(20 + L^2)
solving L using algebra skill
192/L = sqroot(20 + L^2)
square both sides to cancel the sqroot
36864 =L^2( 20 + L^2)
distribute L then
36864 = 20L^2 + L^4
then further simplification and algebra skills like an animal !!
you will get the value of L
L = 13.5 rounded off or 13.5 inches
then substitute L to equation 1
192= (13.5)W
then
W = 14.2222 or 14.2 inches!!
I hope it helps!!!
