Explanation:
Remaining side using pythagorean theorem = sqrt ( 5^2 - 4^2) = 3 cm
area is then 1/2 b * h = 1/2 * 3 * 4 = 6 cm^2
Using Heron's Formula
area = sqrt ( s ( s-a)(s-b)(s-c) ) a = 3 b = 4 c = 5 s = (a+b+c)/2 = 6
area = sqrt ( 6 (6-3)(6-4)(6-5) ) = sqrt ( 6 *3 * 2 *1) = sqrt ( 36 ) = 6 cm^2
(which agrees with the answer we found above)