Answer:
BD=10
Step-by-step explanation:
1. BAD=BCD=60
Therefore
BA = BC (sides opposite to equal angles are equal)
BA=20
Therefore BC=20
2.BAC+BCA+ABC=180(sum of the interior angles of a triangle is 180)
60+60+ABC=180
ABC=180-120
ABC=60
ABC=BAC=BCA
Therefore ABC is an equilateral triangle
AC=20
4. AC=AD+DC
AD=DC (BD is a perpendicular bisector)
Therefore
AC=2×AD
AC=20
20=2×AD
AD=20÷2
AD=10
Hope this helps!
