$$ctg\frac{7\pi}{4}$$
$-1$
$$\cot(\frac{3\pi }{2}+\frac{\pi }{4})=\frac{\cot(\frac{3\pi }{2})\cot(\frac{\pi }{4})-1}{\cot(\frac{\pi }{4})+\cot(\frac{3\pi }{2})}$$
$$\frac{0\cot(\frac{\pi }{4})-1}{\cot(\frac{\pi }{4})+0}$$
$$\frac{0\times 1-1}{1+0}$$
$$-1$$
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