20．在$\triangle ABC$中，内角$A$、$B$、$C$的对边分别是$a$、$b$、$c$，且$a^{2}+b^{2}+\sqrt{2}ab=c^{2}$．
（Ⅰ）求$C$；（Ⅱ）设$\cos A\cos B=\dfrac{3\sqrt{2}}{5}$，$\dfrac{\cos(\alpha+A)\cos(\alpha+B)}{\cos^{2}\alpha}=\dfrac{\sqrt{2}}{5}$，求$\tan\alpha$的值．
Answer key
（Ⅰ）$C=\dfrac{3\pi}{4}$；（Ⅱ）$\tan\alpha=4$ 或 $\tan\alpha=1$