Using the Law of Cosines for both triangle(ABC) and triangle(ABM):
42 = 32 + (2a)2 - 2(3)(2a)cos(B) (2a)2 = a2 + 32 - 2(3)(a)cos(B)
16 = 9 + 4a2 - 12a·cos(B) 4a2 = a2 + 9 - 6a·cos(B)
7 - 4a2 = -12a·cos(B) 3a2 - 9 = -6a·cos(B)
cos(B) = (4a2 - 7) / (12a) cos(B) = (9 - 3a2) / (6a)
Combining: (4a2 - 7) / (12a) = (9 - 3a2) / (6a)
4a2 - 7 = 18 - 6a2
10a2 = 25
a = sqrt(2.5)
BC = 2·sqrt(2.5)
I have never heard of Appollonius's Theorem, but does seem like it can be used here.
Using the information from here: https://en.wikipedia.org/wiki/Apollonius%27s_theorem
32 + 42 = 2( (2a)2 + a2 ) | |
25 = 2( 4a2 + a2 ) |
|
25 = 2( 5a2 ) | |
25 = 10a2 |
|
2.5 = a2 | |
a = √[ 2.5 ] |
|
BC = 2a = 2√[ 2.5 ] |