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## Section 2.1 Side and Angle Relationships

### Subsection Introduction

From geometry we know that the sum of the angle in a triangle is 180°. Space there any relationships between the angle of a triangle and also its sides?

First the all, you have actually probably observed the the longest side in a triangle is constantly opposite the largest angle, and the shortest side is opposite the smallest angle, as portrayed below.

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It is usual to brand the angle of a triangle with capital letters, and the next opposite every angle v the equivalent lower-case letter, as displayed at right. We will certainly follow this exercise unless suggested otherwise.

Example 2.2.

In ( riangle FGH, angle F=48degree,) and also (angle G) is obtuse. Next (f) is 6 feet long. What have the right to you conclude around the various other sides?

Because (angle G) is greater than (90degree ext,) we understand that (angle F +angle G) is greater than (90degree + 48degree = 138degree ext,) so (angle F) is less than (180degree-138degree = 42degree.) Thus, (angle H lt angle F lt angle G,) and also consequently (h lt f lt g ext.) We deserve to conclude that (h lt 6) feet long, and (g gt 6) feet long.

Checkpoint 2.3.

In isosceles triangle ( riangle RST ext,) the vertex angle (angle S = 72degree ext.) Which side is longer, (s) or (t ext?)

### Subsection The Triangle Inequality

It is also true the the sum of the lengths of any type of two sides of a triangle must be higher than the 3rd side, or rather the 2 sides will certainly not fulfill to type a triangle. This reality is referred to as the triangle inequality.

Triangle Inequality.We cannot use the triangle inequality to discover the *exact* lengths of the sides of a triangle, however we can find largest and also smallest possible values for the length.

Two political parties of a triangle have actually lengths 7 inches and 10 inches, as presented at right. What deserve to you say about the size of the third side?

We let (x) stand for the length of the 3rd side that the triangle. By feather at every side in turn, us can use the triangle inequality three various ways, come get

We already know that (x gt -3) due to the fact that (x) need to be positive, but the other two inequalities do provide us brand-new information. The 3rd side have to be better than 3 inches but less 보다 17 customs long.

Checkpoint 2.5.

Can you do a triangle with three wooden rod of lengths 14 feet, 26 feet, and also 10 feet? lay out a picture, and also explain why or why not.

### Subsection best Triangles: The Pythagorean Theorem

In chapter 1 we supplied the Pythagorean organize to derive the distance formula. We can likewise use the Pythagorean organize to uncover one next of a right triangle if we know the various other two sides.

Pythagorean Theorem.In a best triangle, if (c) represents the length of the hypotenuse, and also the lengths that the 2 legs space denoted by (a) and (b ext,) then

Example 2.6.

A 25-foot ladder is placed against a wall surface so that its foot is 7 feet from the base of the wall. How far up the wall does the ladder reach?

We do a sketch of the situation, as displayed below, and label any type of known dimensions. We"ll speak to the unknown height (h ext.)

The ladder creates the hypotenuse that a appropriate triangle, so us can apply the Pythagorean theorem, substituting 25 because that (c ext,) 7 because that (b ext,) and (h) for (a ext.)

eginalign*h^2 + 49 amp = 625 ampamp lert extSubtract 49 native both sides.\h^2 amp = 576 ampamp lert extExtract roots.\h amp = pm sqrt576 ampamp lert extSimplify the radical.\h amp = pm 24endalign*

The height must be a confident number, so the equipment (-24) does not make sense for this problem. The ladder reaches 24 feet increase the wall.

Checkpoint 2.7.

A baseball diamond is a square who sides are 90 feet long. The catcher at residence plate watch a runner on an initial trying to steal second base, and throws the ball to the second-baseman. Discover the straight-line distance from house plate to second base.

Note 2.8.

Keep in mind that the Pythagorean theorem is true just for appropriate triangles, for this reason the converse the the to organize is also true. In various other words, if the sides of a triangle accomplish the connection (a^2 + b^2 = c^2 ext,) then the triangle should be a best triangle. We can use this fact to check whether or not a given triangle has a best angle.

Example 2.9.Delbert is paving a patio in his ago yard, and also would like to understand if the edge at (C) is a right angle.

He measures 20 cm along one next from the corner, and also 48 cm along the other side, put pegs (P) and also (Q) at each position, as shown at right. The line joining those two pegs is 52 centimeter long. Is the edge a appropriate angle?

Checkpoint 2.10.

The sides of a triangle measure 15 inches, 25 inches, and also 30 customs long. Is the triangle a best triangle?

The Pythagorean organize relates the political parties of *right* triangles. However, for information about the political parties of other triangles, the ideal we have the right to do (without trigonometry!) is the triangle inequality. No one does the Pythagorean theorem help us uncover the *angles* in a triangle. In the next section we uncover relationships in between the angles and the political parties of a appropriate triangle.

Review the following skills you will require for this section.

Algebra Refresher 2.2.(displaystyle :x lt 3)

(displaystyle :x le 8)

(displaystyle :x le -1)

(displaystyle :x gt -1)

Positive

Negative

Positive

Negative

Negative

Negative

### Subsection section 2.1 Summary

Subsubsection VocabularyConverse

Extraction the roots

Inequality

Subsubsection ConceptsThe longest next in a triangle is opposite the largest angle, and also the shortest side is the opposite the the smallest angle.

Triangle Inequality: In any kind of triangle, the sum of the lengths of any kind of two political parties is higher than the length of the third side.

Pythagorean Theorem: In a best triangle with hypotenuse (c,~~ a^2 +b^2 = c^2 ext.)

If the political parties of a triangle accomplish the partnership (~a^2 +b^2 = c^2~ ext,) then the triangle is a ideal triangle.

Subsubsection research QuestionsIs it always true the the hypotenuse is the longest side in a best triangle? Why or why not?

In ( riangle DEF ext,) is it feasible that (~d+egt f~) and also (~e+fgt d~) are both true? describe your answer.

In a best triangle with hypotenuse (c ext,) we recognize that (~a^2 +b^2 = c^2~ ext.) Is it also true the (~a + b = c~ ext?) Why or why not?

The two much shorter sides of one obtuse triangle space 3 in and also 4 in. What are the possible lengths because that the third side?

Subsubsection SkillsIdentify inconsistencies in figures #1-12

Use the triangle inequality to placed bounds top top the lengths of political parties #13-16

Use the Pythagorean to organize to uncover the political parties of a best triangle #17-26

Use the Pythagorean theorem to identify right triangles #27-32

Solve difficulties using the Pythagorean to organize #33-42

### Exercises Homework 2.1

Exercise Group.

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For problems 1–12, define why the measurements presented cannot be accurate.