А) х^2 + 4х - 21 = 0; б) 7х^2 - 4х - 3 = 0; в) 2х^2 - 2х - 1 = 0.

a)D=b^2-4ac=16-(-84)=100 k=[latex] \sqrt{100}[/latex]=10 x1=(-k+[latex] \sqrt{k^2-ac} [/latex])/a=(-10+[latex] \sqrt{100+21} [/latex])/1=-10+[latex] \sqrt{121} [/latex]=-10+11=1 x2=(-k[latex] \sqrt{k^2-ac} [/latex])/a=(-10-[latex] \sqrt{100+21} [/latex])/1=-10-[latex] \sqrt{121} [/latex]=-10-11=-21 b)D=16+84=100 k=10 x1=(-10+[latex] \sqrt{100+21} [/latex])/7=(-10[latex] \sqrt{121} [/latex])/7=(-10-11)/7=-3 x2=(-10-[latex] \sqrt{100+21} [/latex])/7=(-10-[latex] \sqrt{121} [/latex])/7=(-10+11)/7=1/7 c)D=4+8=12 k=[latex] \sqrt{12} [/latex]  x1=[latex] (-\sqrt{12} - \sqrt{ \sqrt{12} +2} )/2=(-2 \sqrt{3} - \sqrt{2 \sqrt{3} +2} )/2=(-2 \sqrt{3} - [/latex][latex]\sqrt{2( \sqrt{3}+1 } ))/2[/latex] x2=[latex](-\sqrt{12} +\sqrt{ \sqrt{12} +2} )/2=(-2 \sqrt{3} +\sqrt{2 \sqrt{3} +2} )/2=[latex](-2 \sqrt{3} - \sqrt{2( \sqrt{3}+1 } ))/2[/latex] /