Answer:
Step-by-step explanation:
21.
x²-4x+9=0
[tex]x=\frac{4\pm\sqrt{(-4)^2-4*1*9} }{2*1} \\=\frac{4\pm\sqrt{16-36} }{2} \\=\frac{4\pm2\sqrt{-5} }{2} \\=2\pm\sqrt{5} \iota[/tex]
