In `DeltaABC`, a=9,b=8 and c=4, prove that:
`cosB-2cosC=-4/3`

Explanation

To prove that \( \cos B - 2 \cos C = -\frac{4}{3} \) in triangle \( \Delta ABC \) where \( a = 9 \), \( b = 8 \), and \( c = 4 \), we will use the cosine formulas for angles \( B \) and \( C \).

### Step 1: Calculate \( \cos B \)

The formula for \( \cos B \) is given by:
\[ \cos B = \frac{c^2 + a^2 - b^2}{2ac} \]
...