Given $a=12 \mathrm{~cm}, b=9 \mathrm{~cm}$ and $c=15$, write ratio (in lowest terms) would represent cos A? (remember to write a ratio as top/bottom)

Step 1 ：\(\cos A = \frac{a^2 + b^2 - c^2}{2ab}\)

Step 2 ：\(\cos A = \frac{12^2 + 9^2 - 15^2}{2 \cdot 12 \cdot 9}\)

Step 3 ：\(\cos A = \frac{144 + 81 - 225}{216}\)

Step 4 ：\(\cos A = \frac{0}{216}\)

Step 5 ：\(\boxed{\cos A = 0}\)