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4. Find the area of a triangle two sides of which are 18 cm and 10 cm and the perimeter is 42 cm.
Answer :

Let the sides of a triangle a = 18 cm, b = 10 cm and c
We have, perimeter = 42 cm
So, a + b + c = 42
By substituting the values,
18 + 10 + c = 42
28 + c = 42cm
c = 42 - 28 cm
Thus, c = 14cm
Now, we know that, s=a+b+c2
Therefore, s=18+10+142=422=21cm

Now, Area of triangle = 21(21?18)(21?10)(21?14) ...(Since, Heron's formula [area = s(s?a)(s?b)(s?c)])
= 21×3×11×7
= 7×3×3×11×7
= 2111cm2