\displaystyle x^4-13x^2+36=0x^2=tt^2-13t+36=0D=169-144=25=5^2t_{1.2}= \frac{13\pm 5}{2}t_1=4; t_2=9x^2=4; x=\pm 2 x^2=9; x=\pm 3
\displaystyle x^4-13x^2+36=0x^2=tt^2-13t+36=0D=169-144=25=5^2t_{1.2}= \frac{13\pm 5}{2}t_1=4; t_2=9x^2=4; x=\pm 2 x^2=9; x=\pm 3