Question

the length of three sides of a  triangle are 75cm,70cm and 65cm. finds its area

Answer 1

firstly simply calculate semi-perimeter which is equal to (a+b+c)/2 where a,b,c are the sides of triangles and then put it into the following formula
$\sqrt{s(s-a)(s-b)(s-c)}$ and then solve it then you will get the answer
1567.482057 square units
this formula is called herons formula

Answer 2

s = perimeter/2 = 75+70+65 = 105

from heron's formula,

ar. = √(s(s-a)(s-b)(s-c))
    = √(105(30)(35)(40))
    = 4410000