In $\triangle ABC$, $\angle BCA$ is obtuse and $CD$ is the orthogonal projection of $AC$ on the extended $BC$. Then —

• a$AB^2 = AC^2 + BC^2$
• b$AB^2 = AC^2 + BC^2 + 2 BC \cdot CD$
• c$AB^2 = AC^2 + BC^2 - 2 BC \cdot CD$
• d$AB^2 = AC^2 - BC^2$