Answer:
4 d
Step-by-step explanation:
Given D is mid-point of BC and AD⊥AC
From right angle △DAC
cosC=
a/2
b
=
a
2b
...(i)
also from cosine formulae
cosC=
2ab
a
2
+b
2
−c
2
...(ii)
From (i) and (ii) we get
a
2b
=
2ab
a
2
+b
2
−c
2
⇒4b
2
=a
2
+b
2
−c
2
⇒b
2
=
3
a
2
−c
2
∴cosAcosC=
2bc
b
2
+c
2
−a
2
.
a
2b
=
ca
[(
3
a
2
−c
2
)+c
2
−a
2
]
=
3
2
(
ca
c
2
−a
2
)
