Answer:
The answer is c
Step-by-step explanation:
Let's say David's age right now is x years
x + 6 = 2(x - 7)
x + 6 = 2x - 14
x = 20
Therefor David is 20 years old
By using The half-angle formula
cos X/2 = ±√[(1 + cos X)/ 2]
But we have cos x=2/3
= ±√[(1 + 2/3)/ 2]
= ±√[(5/3 )/ 2]
= ±√[(5/5]
=±√1
Therefore answer is ±1
cos A/2 = ±√[(1 + cos A )/ 2]
