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Problem
One of the roots of the polynomial equation is given. Find the other roots. $1.$ $-2x^{4}+13x^{3}-21x^{2}+2x+8=0$ $x_{1}=-\dfrac {1} {2}$ $2.$ $x^{4}-3x^{2}+2=0$ $x_{1}=1$ $3$ $x^{4}-x^{3}-7x^{2}+13x-6=0$ $x_{1}=1$ $4$ $x^{5}-5x^{4}-3x^{3}+15x^{2}-4x+20=0$ $x_{1}=2$ $5.$ $2x^{4}-17x^{3}+13x^{2}+53x+21=0$ $x_{1}$ $=-1$