найти через дискрименант

1)х2-7х+12=0

D=b2-4ac

D=(-7)2-4×1×12=1

х1=-b±√D/2a

x1=7+1/2×1=4

x2=7-1/2×1=3

2)x2-6x-40=0

D=36-4×1×(-40)=196

√D=14

x1=6+14/2×1=10

x2=6-14/2×1=-4

3)x2+11x+28=0

D=121-4×1×28=9

√D=3

x1=-11+3/2×1=-4

x2=-11-3/2×1=-7

4)5x2-11x+6=0

D=121-4×5×6=1

x1=-11+1/2×5=-1

x2=-11-1/2×5=-6/5

5)8x2+2x-3=0

D=4-4×8×(-3)=100

√D=10

x1=-2+10/2×8=1/2

x2=-2-10/2×8=-3/4