Similar triangles.

(h-x)/h=y/a

Y = a (h-x)/h is obtained.

The area of rectangular EFGH is

S=xy=x[a(h-x)]/h=(-a/h)x？ +ax，∫-a/h & lt； ∴S has a maximum value.

When x=-a/[2(-a/h)]=h/2,

The maximum area of an inscribed rectangle is S=-a? /[4(-a/h)]=ah/4