Find area of a triangle if it's sides are $7\mathrm{cm}$, $10\mathrm{cm}$ and $5\mathrm{cm}$.

Given: Sides of triangle: $7\mathrm{cm}$, $10\mathrm{cm}$, $5\mathrm{cm}$

To find: Perimeter and Area of triangle

Solution: First lets find the perimeter of the triangle. $$\begin{array}{rcll}\because & P& =& a+b+c\\ \therefore & P& =& 7+10+5\\ \therefore & P& =& 22\mathrm{cm}\end{array}$$Therefore, the perimeter of the given triangle is $22\mathrm{cm}$.

Now lets find the semi perimeter of the triangle.
As we know that, semi perimeter is half of the perimeter,

$$\begin{array}{rcll}\because & s& =& \frac{P}{2}\\ \therefore & s& =& \frac{22}{2}\\ \therefore & s& =& 11\mathrm{cm}\end{array}$$

Now, as per the Heron’s formula, we know;

$$\begin{array}{rcl}\text{Area of Triagnle}& =& \sqrt{s(s-a)(s-b)(s-c)}\\ & =& \sqrt{11(11-7)(11-10)(11-5)}\\ & =& \sqrt{11\times 4\times 1\times 6}\\ & =& \sqrt{2^2\times 66}\\ & =& 2\times \sqrt{66}\\ & =& 2\sqrt{66}{\mathrm{cm}}^2\end{array}$$Therefore, the area of the given triangle is $2\sqrt{66}{\mathrm{cm}}^2$.