The rectangle is inscribed in a right angled triangle.
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and
are the side of the rectangle.
The sides of the triangle are
and
.
Two points form of a line equation is
.
The two points are
and
is

The area of the rectangle is
.

The area of the rectangle is maximum when its first derivative becomes zero.
\Apply derivative on each side with respect to
.

\
Equate
to zero.

The length of the rectangle is
.
Substitute
in
.

Therefore the area of the largest rectangle that is inscribed in a right angled triangle is
.
\ \
The area of the largest rectangle that is inscribed in a right angled triangle is
.