Cosine Rule (missing angle)
a
2
=
b
2
+
c
2
−
2
b
c
C
o
s
A
∴
C
o
s
A
=
b
2
+
c
2
−
a
2
2
b
c
Find x° (to 2 d.p.)
12
17
7
x°
c
o
s
x
=
12
2
+
7
2
−
17
2
2
(
12
)
(
7
)
=
−
0.57
S
o
c
o
s
−
1
(
−
0.57
)
=
x
⟹
x
=
124.75
∘