যদি, $\cos x+\cos y=a$ এবং $\sin x+\sin y=b$ হয়, তবে $\cos (x+y)$ এর মান কোনটি?

$\frac{a-b}{a+b}$

$\frac{a^{2}-b^{2}}{a^{2}+b^{2}}$

$\frac{2 a^{2}-b^{2}}{a^{2}+b^{2}}$

$\frac{a^{3}+2 b^{2}}{a^{3}-b^{2}}$

$\frac{a^{2}+3 b^{2}}{a^{2}-2 b^{2}}$