Answer:
A: 15
Step-by-step explanation:
The angles are opposite to each other.
Vertically opposite angles are equal in size.
Put up an equation and solve for a.
6a + 10 = 3a + 55
Subtract 3a and 10 on both sides.
6a - 3a = 55 - 10
Combining like terms.
3a = 45
Divide both sides by 3.
a = 45/3
a = 15
The value of a is 15.
The drawing shows that angles BAD and CAD are congruent, which means that the line segment AD bisects the angle BAC. By the angle bisector theorem,

But AB and AC are also shown to be congruent, so the right hand side reduces to 1, and we have
, which means CD and BD are also congruent.
AD is congruent to itself and happens to be a part of two triangles, ABD and ACD.
So by the side-side-side postulate, the triangles ABD and ACD are congruent, which means corresponding parts (sides and angles) are congruent to each other. And because triangles ABD and ACD are congruent, the "total" triangle ABC is isosceles, and AD must be perpendicular to BC. This last observation means that
.
In any triangle, the interior angles must sum to 180 degrees, so we have

the area and circumference ones are correct. a quadrilateral has 4 sides, so that one is incorrect. hope this helps!
Option D
∠1 = ∠8
Step-by-step explanation:
If two parallel lines are cuts by transversal ,then the alternate interior angles formed are congruent.
∠1 = ∠6 (since they are vertically opposite angles)
∠6=∠3 (Since alternate interior angles are congruent)
∠3=∠8 (since they are vertically opposite angles)
Therefore ∠1 = ∠8