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