Find the area of triangle ABC if AB = BC = 12 and angle ABC = 150 degrees.
Since AB = BC triangle ABC is isoceles and angles BAC and BCA are equal
Since ABC = 150o and the sum of the interior angles of a triangle is 180o
then angles A and C total 30o and therefore each of them is 15o
Draw a perpendicular from the apex B down to a point on side AC - call the point P
BP
Using the sine function: sin(15) = ———
12
BP = (12)(0.65029) = 7.80 this is the height of the triangle
AP
Using the cosine function: cos(15) = ———
12
AP = (12)(0.96593) = 11.59 this is half the base of the triangle
So the whole base of the triangle is (2)(11.59) = 23.18 (We could have skipped this step. Do you know why?)
1
The formula for the area of a triangle is — • base • height
2
So, the area of this triangle is 0.5 • 23.18 • 7.80 = 90.40 units2
The answer in the back of the book might be slightly different, because of the rounding that happened here.
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