$\"\"$

You know the measures of two sides and their included angle, so use the

Law of cosines to find the unknown angle measure.

In $\"\"$ , by the law of cosines, $\"\"$

Substitute $\"\"$

$\"\"$

$\"\"$ (Evaluate powers)

$\"\"$ (Multiply)

$\"\"$ (Add) $\"\"$

$\"\"$ (Subtrct 4.25 from each side)

$\"\"$ (Divide each side by $\"\"$ )

Solve for R.

R equals the inverse cosine of $\"\"$ .

$\"\"$

$\"\"$ (Use a calculater) $\"\"$

The unknown angle measure $\"\"$ .