math
posted by lisa .
one side of an equilateral triangle is 42cm.find the length of an altitude of the triangle.

You can use the Pythagorean Theorem:
a^2 + b^2 = c^2
a^2 + 21^2 = 42^2
a^2 + 441 = 1764
a^2 = 1764  441
a^2 = 1323
a = 36.37 cm