All sides are equal in an equilateral triangle and the perpendicular bisects the side too.
so BC will be bisected and the segments be 5 each.
you can use Pythagoras theorem to find the altitude length, hypotenuse will be one side of the triangle.
$h^2=100-25=75$
$h=\sqrt{75}=5\sqrt3$
Please consider the complete question.
The base of a solid oblique pyramid is an equilateral triangle with a base edge length of 18 inches. What is the height of the triangular base of the pyramid?
First of all, we will draw an equilateral triangle.
Since the given triangle is equilateral triangle, so all sides will have same length that is 18 inches.
We know that height of an equilateral triangle is , where a is side length of equilateral triangle.
Therefore, the height of the given equilateral triangular base will be inches and option B is the correct choice.
