BCD is a straight line. ABC is a triangle.
Show that triangle ABC is an isosceles triangle.
Give a reason for each stage of your working.

It’s an isosceles triangle because angle A is equal to angle C (63deg). This is calculated by 180 - 117 = 63 degrees.
Using the exterior angle is equal to the interior angles property, angle A is equal to 117 - 54 (which is 63).
Because these two angles are equal to eachother, the triangle is isosceles.
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