Quadrilateral abcd is inscribed in a circle. m∠a is 64°, m∠b is (6x + 4)°, and m∠c is (9x − 1)°. what is m∠d?
a.
64°
b.
82°
c.
90°
d.
98°
e.
116°

The m∠D is 98° ⇒ answer D

Step-by-step explanation:

* Lets revise some facts in the circle

- The quadrilateral is inscribed in a circle if its four vertices lie on the

  circumference of the circle

- It is called a cyclic quadrilateral

- Every two opposite angles in it are supplementary means the

  sum of their measures is 180°

∵ ABCD is inscribed in a circle

∴ ABCD is a cyclic quadrilateral

∵ ∠A and ∠C are opposite angles in the cyclic quadrilateral ABCD

∴ ∠A and ∠C are supplementary

∴ m∠A + m∠C = 180°

∵ m∠A = 64°

∵ m∠C = (9x - 1)°

∴ 64 + (9x - 1) = 180 ⇒ simplify

∴ 63 + 9x = 180 ⇒ subtract 63 from both sides

∴ 9x = 117 ⇒ divide both sides by 9

∴ x = 13

- Lets find the measure of ∠B

∵ m∠B = (6x + 4)°

∵ x = 13

∴ m∠B = 6(13) + 4 = 78 + 4 = 82°

- Lets find the measure of ∠D

∵ ∠B and ∠D are opposite angles in the cyclic quadrilateral ABCD

∴ ∠B and ∠D are supplementary

∴ m∠B + m∠D = 180°

∵ m∠B = 82°

∴ 82° + m∠D = 180° ⇒ subtract 82° from both sides

∴ m∠D = 98°

* The m∠D is 98°

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