令x = y+3则 (y+3)³ - 9(y+3)² + 8(y+3) + 30 =0y³ + 9y²+ 27y+27 - 9y² - 54y - 81 + 8y + 24+30 =0y³ - 19y =0y=0 或 y= ±√19∴x = 3,3±√19y=0 或 y= ±√19这个怎么得来的y³ - 19y=0 y(y² -19)=0 y=0或y²-19=0