[4] what is the largest possible area for a right triangle whose hypotenuse is 5cm long, and what are its dimensions? 25

Let the base and height of the right triangle be b and h respectively.

Then the area of the triangle (which is to be maximized) is

A = bh/2.  Draw the triangle with the right angle positioned at the origin, in the first quadrant.

We are told that the hypotenuse is 5 cm.  Then 

b^2 + h^2 = 25 cm^2 must be true.  Alternatively, b = sqrt(25-h^2).

We want to meximize the area of this triangle.  In other words, maximize

A = (b)(h)/2, or, equivalently, maximize A = [sqrt(25-h^2)](h)/2.

Maximize A by differentiating the above formula for A with respect to h and finding the critical value for h.  Once you have that h value, calculate the corresponding b value and find the max area via    A = bh/2.