Pour \omega = \omega_{c}
\Rightarrow \underline{H}(j \omega_{c}) = \frac{1}{1+j} ( 1 point ) or 1+j = \sqrt{2} e^{j \tfrac{\pi}{4}}
d'où
\underline{H}(j \omega_{c}) = \frac{1}{\sqrt{2}} e^{-j\tfrac{\pi}{4}} ( 4 points ) avec \arrowvert\underline{H}\arrowvert = \frac{1}{\sqrt{2}} = (2)^{-\tfrac{1}{2}} ( 1 point )
donc
H_{\textrm{db}} = 20 \log \arrowvert\underline{H}\arrowvert = 20 \log(2)^{-\tfrac{1}{2}}
H_{\textrm{db}} = -10 \log 2 = -3 \textrm{dB} ( 4 points )