X₁+x₂= - b - теорема Виета
x₁x₂=c
x²-9x+20=0
x₁+x₂=9
x₁x₂=20
x₁=5
x₂=4
x²+11x-12=0
x₁+x₂=-11
x₁x₂=-12
x₁= -12
x₂=1
x²+x-56 =0
x₁+x₂=-1
x₁x₂=-56
x₁= -8
x₂=7
D=b²-4ac
x= \frac{-b+- \sqrt{D} }{2*a}x=2∗a−b+−D
x²-19x+88=0
D=(-19)*4*88*1=361-352=9=3²
\begin{lgathered}x_{1}= \frac{-(-19)- \sqrt{9} }{2*1} = \frac{19-3}{2} = \frac{16}{2} =8 \\ x_{2}= \frac{-(-19)+ \sqrt{9} }{2*1} = \frac{19+3}{2} = \frac{22}{2} =11 \\\end{lgathered}x1=2∗1−(−19)−9=219−3=216=8x2=2∗1−(−19)+9=219+3=222=11