[tex]cos^2A+cos^2B=0.5-\sqrt{2}cosAcosB \\\ cos^2A+cos^2(\frac{3\pi}{4}-A)=0.5-\sqrt{2}cosAcos(\frac{3\pi}{4}-A) \\\ \frac{1+cos2A}{2}+\frac{1+cos(\frac{3\pi}{2}-2A)}{2}=0.5-\sqrt{2}cosA(cos\frac{3\pi}{4}cosA+sin\frac{3\pi}{4}sinA) [/tex]
[tex]\\\ 1+cos2A+1+cos(\frac{3\pi}{2}-2A)=1-2\sqrt{2}cosA(-\frac{\sqrt{2}}{2}cosA+\frac{\sqrt{2}}{2}sinA) \\\ 2+cos2A-sin2A=1+2cos^2A-2sinAcosA \\\ 1+cos2A-sin2A=\frac{2(1+cos2A)}{2}-sin2A \\\ 1+cos2A=1+cos2A[/tex]
Верно, значит и исходное равенство верно.
