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Drawing the bisector of vertical angle 120 degree, it bisect the base of isosceles triangle and is perpendicular to the base.
The altitude is the leg of the right triangle across 30 degree angle and as we know is half of the hypotenuse's length.
Let x the length of hypotenuse, then the length of the leg is x/2 cm.
Applying Pythagorean theorem we have:
,solving this equation
cm, and the length of the altitude is: