Inequalities

Applications of Inequalities to Angles

Problems

Problems

Inequalities are useful in many situations. In particular, they are useful in geometry when classifying angles.. There are three types of angles: right angles, acute angles, and obtuse angles. Right angles have a measure of exactly 90 degrees. Acute angles have a measure of less than 90 degrees. Obtuse angles have a measure of greater than 90 degrees (but not more than 180 degrees).

Thus, we can write out inequalities classifying the three types of angles:

x = the measure of angle A in degrees
If x < 90 , then A is an acute angle.
If x = 90 , then A is a right angle.
If x > 180 , then A is an obtuse angle.

Example 1: Angle A measures x degrees. Is A acute if x = 15 ? If x = 65 ? If x = 90 ? If x = 135 ?

15 < 90 ? Yes. A is acute if x = 15 .
65 < 90 ? Yes. A is acute if x = 65 .
90 < 90 ? No. A is not acute if x = 90 .
135 < 90 ? No. A is not acute if x = 135 .

Example 2: If angle A measures 2x - 5 degrees, for which of the following values of x is A obtuse? {25, 45, 65, 85}

2(25) - 5 > 90 ? No.
2(45) - 5 > 90 ? No.
2(65) - 5 > 90 ? Yes.
2(85) - 5 > 90 ? Yes.
Thus, A is obtuse for x = {65, 85} .

Example 3: Which angle is right? Acute? Obtuse?


Angles
Angle A is acute, angle B is right, and angle C is obtuse.