Property: A right-angled triangle with maximum area has to be an isosceles right-angled triangle.
Thus,
Let x and y be the two equal arms of such a triangle:
x = y = 40/2 = 20
Area will be maximum when the two legs are of same length. this is because of the property that the product of any two numbers is maximum when both are equal, only if the addition of those two numbers is a constant. hence 40= 20+20. thus 20,20 are two sides and area is 200