[tex](1+tgx)^{2}+(1-tgx)^{2}=1+2tgx+tg^{2}+1-2tgx+tg^{2}x=2+2tg^{2}x=2(1+tg^{2}x)=\frac{2}{cos^{2} x}[/tex]
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[tex](1+tgx)^{2}+(1-tgx)^{2}=1+2tgx+tg^{2}+1-2tgx+tg^{2}x=2+2tg^{2}x=2(1+tg^{2}x)=\frac{2}{cos^{2} x}[/tex]