It's an isosceles triangle so angles A and BCA are congruent.
Angle BCA is the supplement of BCD, so 180-109 = 71.
Angle A is congruent to that, so A=71 degrees.
Let's see if we can get that in the format they want, kind of as a proof.
1. ∠BCD=109° Reason: Given
2. AB ≅ BC Reason: Given
3. ∠BCA = 71° Reason: Linear pairs are supplementary
4. ΔABC is isosceles. Reason: Definition of isosceles
5. ∠A ≅ ∠BCA Reason: Isoceles triangle theorem
6. ∠A = 71° Reason: Def congruent
Answer: 71 degrees
The correct answer for the completion exercise shown above is: sine.
Therefore, the complete text is shown below: "<span>In a right triangle, the sine of an angle can be found by dividing the length of the opposite leg by the length of the triangle's hypotenuse".
</span>
A right triangle is a triangle that has an angle of 90 degrees.
The sine is one of the most common trigonometric functions. Therefore, you have that the sine of an angle is:
Sin(α)=opposite/hypotenuse
