In triangle ABC, angle C = 45 °, AD bisector, CAD = 30 °. Find Angle B.
August 25, 2021 | education
| The bisector of an angle is a ray starting at the apex of the angle that divides the angle into two equal parts.
The bisector AD divides the ΔABC triangle into two triangles ΔABD and ΔADC.
Consider a triangle ΔADC.
Let’s calculate the degree measure of the angle ∠ADC. Since the sum of all the angles of the triangle is 180º, then:
∠ADС = 180º – ∠DСА – ∠DАС;
∠ADC = 180º – 45º – 30º = 105º.
Since the angle ∠ВDC is a deployed angle (equal to 180º), then:
∠ВDA = 180º – ADС;
∠ВDA = 180º – 105º = 75º.
Since the bisector of an angle divides it in half, then:
∠ВАD = ∠DАС = 30º.
Since the sum of all the angles of the triangle is 180º, then:
∠АВD = 180º – 30º – 75º = 75º.
Answer: the degree measure of the angle ∠В is equal to 75º.
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