Mirror formula relating object distance u, image distance v and focal length f is?

• a$\frac{1}{u}-\frac{1}{v}=\frac{1}{f}$
• b$\frac{1}{u}+\frac{1}{v}=\frac{1}{f}$
• c$u+v=f$
• d$u\cdot v=f$