Table of Contents
Last modified on August 3rd, 2023
SAS means ‘Angle-Side-Angle’. SAS triangles are triangles where two sides and the angle between them are known. Shown below is a SAS triangle, △ABC, where b and c are the two known sides, while ∠B is the known, common angle present between the two sides.
It involves three steps:
Step 1: Use the Law of Cosines to calculate the missing side
Step 2: Use The Law of Sines to find the smaller of the two unknown angles
Step 3: Use the angle sum rule of a triangle to find the last angle
Let us take some examples to understand the concept better.
Find the missing angles and the side in the given SAS triangle.
In the triangle, the given angles and side is:
Side b = 4
Side c = 6
∠A = 45°
Using theLaw of Cosines, we will calculate the missing side, side a
a2 = b2 + c2 − 2bc cos A, here b = 4, c = 6, ∠A = 45°
a2 = 42 + 62 – 2 x 4 x 6 x cos 45°
a2 = 16 + 36 – 48 x cos 45°
a2 = 52 – 48 x 0.70
a2 = 52 – 33.6
a2 = 18.4
a = 4.28
Now, we use The Law of Sines to find the smaller of the two unknown angles
Why the Smaller Angle to be Determined First?
The smaller angle is determined first because the inverse sine function gives answers less than 90° even for angles greater than 90°. By choosing the smaller angle a triangle cannot have two angles greater than 90°.
a/sin A = b/sin B, here ∠A = 45°, a = 4.28, b = 4
4.28/sin 45° = 4/sin B
sin B = 4 x sin 45°/4.28
sin B = 4 x 0.70/4.28
sin B = 0.65
B = sin -1 (0.65)
B = 40.54°
Finally, we will use the angle sum rule of a triangle to find the last undetermined angle, ∠C
∠A + ∠B + ∠C = 180°, here ∠A = 45°, ∠B = 40.54°
45° + 40.54° +∠C = 180°
∠C = 180° – (45° + 40.54°)
∠C = 94.46°
Find the missing angles and the side in the given triangle.
The given triangle is an SAS triangle.
Side a = 6.8
Side b = 4.2
∠C = 110°
Using theLaw of Cosines, we will calculate the missing side, side c
c2 = a2 + b2 − 2ab cos C, here a = 6.8, b = 4.2, ∠C = 110°
c2 = 6.82 + 4.22 – 2 x 6.8 x 4.2 x cos 110°
c2 = 46.24 + 17.64 – 57.12 x (-0.34)
c2 = 63.88 + 19.42
c2 = 83.3
c = 9.12
Now, for calculating the Law of Sines, we don’t need to look for the smaller angle as ∠C is greater than 90°, so ∠A and ∠B must be less than 90°.
a/sin A = c/sin C, here, a = 6.8, c = 9.12, ∠C = 110°
6.8/sin A = 9.12/ sin 110°
sin A = 6.8 x sin 110°/9.12
sin A = 6.8 x 0.93/9.12
sin A = 0.69
A = sin-1 (0.69)
∠A = 43.63°
Finally, we will use the angle sum rule of a triangle to find the last undetermined angle, ∠B
∠A + ∠B + ∠C = 180°, here ∠A = 43.63°, ∠C = 110°
43.63° + ∠B + 110° = 180°
∠B = 180° – (43.63° + 110°)
∠B = 26.37°
Let us take an example to understand the concept.
To prove: ΔABO ≅ ΔCDO
|1.||AB ≅ CD AB || CD||Given|
|2.||∠BAO ≅∠DCO||Alternate interior angles|
|3.||∠ABO ≅∠CDO||Alternate interior angles|
|4.||ΔABO ≅ ΔCDO||SAS postulate (Hence proved)|
Identify which pair of triangles below illustrates an SAS relationship to prove the triangles are congruent.
Option (b). Here the triangles ΔHIJ and ΔKLM are congruent by the SAS relationship.
Prove whether ΔABC and ΔDEC are congruent by SAS. Given that, BC ≅ DC and C is the midpoint of AE.
Given that BC ≅ DC and C is the midpoint of AE
Now, according to the ‘Vertical Angles Theorem‘ ∠ACB ≅∠ECD
Again, AC ≅ EC as the midpoint C bisects AE into two congruent parts AC and EC
Hence ΔABC ≅ ΔDEC by SAS postulate (Proved)
If ΔABC is an SAS triangle with side lengths a, b and c, then we can find its area with one of the three formulas given below:
The method discussed above is just an extension of the general formula for determining the area of a triangle.
As we know, the general formula of area of a triangle is given by,
Area (A) = ½ x a x h, here a = base, h = height
But, if we are not given the height but we are given side b and angle C in the right triangle, ΔCDA
Using trigonometry, we can write
sin C = h/b
=> h = b sin C
Substituting the value of h in the general formula of area of a triangle, we get,
Area (A) = ½ x a x b sin C = ½ ab sin C
Let us solve an example using the above formula.
Find the area of the given triangle.
As the given triangle is an SAS triangle, we will use the formula
Area (A) = ½ ab sin C, here a = 10, b = 16, ∠C = 55°
= ½ 10 x 16 sin 55°
= 64.8 square units
Last modified on August 3rd, 2023