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a)
x^3+2x^2+5x=0
x\left(x^2+2x+5\right)=0
x=0\ tai\ x^2+2x+5=0
x=0\ tai\ x=-1\pm2i
b)
x^3+x^2+2x-4=0
x=-1\pm\sqrt{3}\cdot i\ tai\ x=1