Answer:
Option B (AB = 2.2 units).
Step-by-step explanation:
The diagram shows that there are two sides given and one angle is given. Therefore, the sine rule must be used to solve the question. The sine rule can be written as:
sin ABC / AC = sin BAC / BC.
Plugging ABC=72 degrees, AC=2.5, and BC=2.1 in the sine rule gives:
sin 72 / 2.5 = sin BAC / 2.1.
Cross multiplying gives:
sin BAC = (2.1*sin 72)/2.5.
sin BAC = 0.79888747368.
Taking sin inverse on both sides gives:
BAC = arcsin (0.79888747368) = 53.0239949 degrees.
To find AB, first, the angle ACB is required. To find that angle, use the triangular law of angles. All the three angles sum up to 180 degrees. Therefore ACB = 180 - 72 - 53.0239949 = 54.9760051 degrees.
Now applying the Sine Rule to find AB:
sin ACB / AB = sin ABC / AC.
sin (54.9760051) / AB = sin 72 / 2.5.
AB = (2.5*sin (54.9760051))/sin (72) = 2.2 (to the nearest tenth).
Therefore, AB = 2.2 units, i.e. Option B!!!
