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# 5.5: Altitudes

Difficulty Level: At Grade Created by: CK-12
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### Altitudes

An altitude is a line segment in a triangle from a vertex and perpendicular to the opposite side, it is also known as the height of a triangle. All of the red lines are examples of altitudes:

As you can see, an altitude can be a side of a triangle or outside of the triangle. When a triangle is a right triangle, the altitude, or height, is the leg. To construct an altitude, construct a perpendicular line through a point not on the given line. Think of the vertex as the point and the given line as the opposite side.

#### Investigation: Constructing an Altitude for an Obtuse Triangle

Tools Needed: pencil, paper, compass, ruler

1. Draw an obtuse triangle. Label it ABC\begin{align*}\triangle ABC\end{align*}, like the picture to the right. Extend side AC¯¯¯¯¯¯¯¯\begin{align*}\overline{AC}\end{align*}, beyond point A\begin{align*}A\end{align*}.
2. Construct a perpendicular line to AC¯¯¯¯¯¯¯¯\begin{align*}\overline{AC}\end{align*}, through B\begin{align*}B\end{align*}.

The altitude does not have to extend past side AC¯¯¯¯¯¯¯¯\begin{align*}\overline{AC}\end{align*}, as it does in the picture. Technically the height is only the vertical distance from the highest vertex to the opposite side.

As was true with perpendicular bisectors, angle bisectors, and medians,the altitudes of a triangle are also concurrent. Unlike the other three, the point does not have any special properties.

Orthocenter: The point of concurrency for the altitudes of triangle.

Here is what the orthocenter looks like for the three triangles. It has three different locations, much like the perpendicular bisectors.

Acute Triangle Right Triangle Obtuse Triangle
The orthocenter is inside the triangle. The legs of the triangle are two of the altitudes. The orthocenter is the vertex of the right angle. The orthocenter is outside the triangle.

#### Identifying Altitudes

Which line segment is an altitude of ABC\begin{align*}\triangle ABC\end{align*}?

In a right triangle, the altitude, or the height, is the leg. If we rotate the triangle so that the right angle is in the lower left corner, we see that leg BC\begin{align*}BC\end{align*} is an altitude.

#### Determining Location of the Orthocenter

1. A triangle has angles that measure 55,60,\begin{align*}55^\circ, 60^\circ,\end{align*} and 65\begin{align*}65^\circ\end{align*}. Where will the orthocenter be found?

Because all of the angle measures are less than 90\begin{align*}90^\circ\end{align*}, the triangle is an acute triangle. The orthocenter of any acute triangle is inside the triangle.

2. A triangle has an angle that measures 95\begin{align*}95^\circ\end{align*}. Where will the orthocenter be found?

Because 95>90\begin{align*}95^\circ > 90^\circ\end{align*}, the triangle is an obtuse triangle. The orthocenter will be outside the triangle.

### Examples

#### Example 1

True or false: The altitudes of an obtuse triangle are inside the triangle.

Every triangle has three altitudes. For an obtuse triangle, at least one of the altitudes will be outside of the triangle, as shown in the picture at the beginning of this section.

#### Example 2

Draw the altitude for the triangle shown.

The triangle is an acute triangle, so the altitude is inside the triangle as shown below so that it is perpendicular to the base.

#### Example 3

Draw the altitude for the triangle shown.

### Review

Write a two-column proof.

1. Given: Isosceles ABC\begin{align*}\triangle ABC\end{align*} with legs AB¯¯¯¯¯¯¯¯\begin{align*}\overline{AB}\end{align*} and AC¯¯¯¯¯¯¯¯\begin{align*}\overline{AC}\end{align*} BD¯¯¯¯¯¯¯¯DC¯¯¯¯¯¯¯¯\begin{align*}\overline{BD} \bot \overline{DC}\end{align*} and CE¯¯¯¯¯¯¯¯BE¯¯¯¯¯¯¯¯\begin{align*}\overline{CE} \bot \overline{BE}\end{align*} Prove: BD¯¯¯¯¯¯¯¯CE¯¯¯¯¯¯¯¯\begin{align*}\overline{BD} \cong \overline{CE}\end{align*}

For the following triangles, will the altitudes be inside the triangle, outside the triangle, or at the leg of the triangle?

1. JKL\begin{align*}\triangle JKL\end{align*} is an equiangular triangle.
2. MNO\begin{align*}\triangle MNO\end{align*} is a triangle in which two the angles measure 30\begin{align*}30^\circ\end{align*} and 60\begin{align*}60^\circ\end{align*}.
3. PQR\begin{align*}\triangle PQR\end{align*} is an isosceles triangle in which two of the angles measure 25\begin{align*}25^\circ\end{align*}.
4. STU\begin{align*}\triangle STU\end{align*} is an isosceles triangle in which two angles measures 45\begin{align*}45^\circ\end{align*}.

Given the following triangles, which line segment is the altitude?

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Date Created:
Jul 17, 2012
Aug 02, 2016
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