The perimeter of an isosceles triangle is 18 and the lateral side is 5. Find the area of the triangle

Let us find out how many conventional units will determine the base of such a triangle, if from the condition of the task we know that with a perimeter of 18 conventional units, its lateral side is 5 conventional units:

18 – 5 * 2 = 18 – 10 = 8.

Let us determine what the area of our triangle will equal, for which we use the well-known formula of Heron:

The semi-perimeter will be equal to – 18: 2 = 9;

√ (9 * (9 – 8) * (9 – 5) * (9 – 5)) = √ (9 * 1 * 5 * 5) = 15.

Answer: 15 square conventional units.