The perimeter of an isosceles triangle is 18cm. What are its sides equal to if the base is 8cm?

The perimeter of a triangle is the sum of the lengths of all its sides, we write the formula:
P = a + b + c.
An isosceles triangle has two sides equal – by definition.
Therefore, if the perimeter and length of the side lying at the base are known (while the side ones are equal), we derive the formula:
P = 2a + c;
Substitute the data we know and find the double product of equal sides:
2a = P – c;
2a = 18 – 8;
2a = 10.
Now let’s find the length of one side:
a = 10: 2;
a = 5.
Thus, the sides are equal to 5 centimeters.
Answer: a = 5 centimeters; h = 5 centimeters.