How to find an angle in an acute / obtuse triangle
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ACT Math › How to find an angle in an acute / obtuse triangle
In a given triangle, the angles are in a ratio of 1:3:5. What size is the middle angle?
Explanation
Since the sum of the angles of a triangle is 
If the smallest angle is 20 degrees, then given that the middle angle is in ratio of 1:3, the middle angle would be 3 times as large, or 60 degrees.
In the triangle below, AB=BC (figure is not to scale) . If angle A is 41°, what is the measure of angle B?
A (Angle A = 41°)
B C
41
82
90
98
Explanation
If angle A is 41°, then angle C must also be 41°, since AB=BC. So, the sum of these 2 angles is:
41° + 41° = 82°
Since the sum of the angles in a triangle is 180°, you can find out the measure of the remaining angle by subtracting 82 from 180:
180° - 82° = 98°
Points A, B, and C are collinear (they lie along the same line). The measure of angle CAD is 
Find the measure of 
Explanation
The measure of 
Because the measures of the three angles in a triangle must add up to 
Two interior angles in an obtuse triangle measure 
Explanation
Interior angles of a triangle always add up to 180 degrees.
Points A, B, C, D are collinear. The measure of ∠ DCE is 130° and of ∠ AEC is 80°. Find the measure of ∠ EAD.
50°
60°
70°
80°
Explanation
To solve this question, you need to remember that the sum of the angles in a triangle is 180°. You also need to remember supplementary angles. If you know what ∠ DCE is, you also know what ∠ ECA is. Hence you know two angles of the triangle, 180°-80°-50°= 50°.