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why obtuse triangles have two exterior altitudes?

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Altitude (triangle)

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"Orthocenter" and "Orthocentre" redirect here. For the orthocentric system, see Orthocentric system.

In geometry, an altitude of a triangle is a line segment through a vertex and perpendicular to (i.e., forming a right angle with) a line containing the base (the side opposite the vertex). This line containing the opposite side is called the extended base of the altitude. The intersection of the extended base and the altitude is called the foot of the altitude. The length of the altitude, often simply called "the altitude", is the distance between the extended base and the vertex. The process of drawing the altitude from the vertex to the foot is known as dropping the altitude at that vertex. It is a special case of orthogonal projection.

The three altitudes of a triangle intersect at the orthocenter, which for an acute triangle is inside the triangle.

Altitudes can be used in the computation of the area of a triangle: one half of the product of an altitude's length and its base's length equals the triangle's area. Thus, the longest altitude is perpendicular to the shortest side of the triangle. The altitudes are also related to the sides of the triangle through the trigonometric functions.

In a right triangle, the altitude from each acute angle coincides with a leg and intersects the opposite side at (has its foot at) the right-angled vertex, which is the orthocenter.

In an isosceles triangle (a triangle with two congruent sides), the altitude having the incongruent side as its base will have the midpoint of that side as its foot. Also the altitude having the incongruent side as its base will be the angle bisector of the vertex angle.

It is common to mark the altitude with the letter h (as in height), often subscripted with the name of the side the altitude is drawn to.

In a right triangle, the altitude drawn to the hypotenuse c divides the hypotenuse into two segments of lengths p and q. If we denote the length of the altitude by hc, we then have the relation

{\displaystyle h_{c}={\sqrt {pq}}} h_c=\sqrt{pq} (Geometric mean theorem)

The altitudes from each of the acute angles of an obtuse triangle lie entirely outside the triangle, as does the orthocenter H.

For acute and right triangles the feet of the altitudes all fall on the triangle's sides (not extended). In an obtuse triangle (one with an obtuse angle), the foot of the altitude to the obtuse-angled vertex falls in the interior of the opposite side, but the feet of the altitudes to the acute-angled vertices fall on the opposite extended side, exterior to the triangle. This is illustrated in the adjacent diagram: in this obtuse triangle, an altitude dropped perpendicularly from the top vertex, which has an acute angle, intersects the extended horizontal side outside the triangle.

Orthocenter

Orthic triangle

Some additional altitude theorems

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why obtuse triangles have two exterior altitudes?​

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why obtuse triangles have two exterior altitudes?