Prove that the perimeter of a triangle is greater than sum of its three altitudes.
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Let there be a triangle ABC with its altitudes D, E, and F from vertices A, B and C respectively.
The altitudes form a right angle at their corresponding bases. Also, in a right triangle the hypotenuse is the longest side. Taking the right triangles formed by the altitudes and the sides as the hypotenuse, we observe that in each triangle, the side forms the longest side, i.e,
In triangle ABD, AB is the longest side
In triangle ACF, AC is the longest side
In triangle CBE, BC is the longest side
So, adding all the three, we get that the perimeter of a triangle is greater than the sum of its three altitudes.