Question 79392
Use the formula for the Pythagorean Theorem to solve because the television screen forms two right triangles.
a^2+b^2=c^2
a=one side = x
b=the other side = x [because the TV screen is a square -- all sides are equal]
c=the hypotenuse = the diagonal = 18
.
Plug-in the values and solve:
 a^2+b^2=c^2  
x^2+b^2=18^2
2x^2=18^2
2x^2=324
2x^2/2=324/2
x^2=162
{{{(sqrt(x^2))}}} = {{{(sqrt(162^2))}}}
x=12.7279=12.73
.
Check by plugging all of the values back into the original formula:
a^2+b^2=c^2
(12.7279)^2+(12.729)^2=18^2
324.054=324 [close-enough]