Find the area of a triangle whose sides are 3 cm, 4 cm and 5 cm.

Consider a = 3 cm, b = 4 cm and c = 5 cm

We know that

Semi perimeter (s) = $\dfrac{(a + b + c)}{2}$

Substituting the values we get,

s = $\dfrac{(3 + 4 + 5)}{2} = \dfrac{12}{2}$ = 6 cm.

Area of triangle = $\sqrt{s(s - a)(s - b)(s - c)}$

Substituting values we get,

$A = \sqrt{6(6 - 3)(6 - 4)(6 - 5)} \\[1em] = \sqrt{6 \times 3 \times 2 \times 1} \\[1em] = \sqrt{36} \\[1em] = 6 \text{ cm}^2.$

Hence, area of triangle = 6 cm².