题目
Problem
The equation of a curve is 2x2−kxy+2=0 and the equation of a line is y=px+3, where k and p are constants.
(a) Given that k=2 and p=11, find the coordinates of the points of intersection of the curve and the line.
[4]
(b) Given instead that p=4, find the set of values of k for which the curve and the line do not intersect.
[5]
题目中文翻译
一条曲线的方程为 2x2−kxy+2=0,一条直线的方程为 y=px+3,其中 k 和 p 为常数。
(a) 已知 k=2、p=11,求曲线与直线交点的坐标。
[4]
(b) 改为已知 p=4,求使曲线与直线不相交的 k 的取值范围。
[5]
解答
(a)
解法一
思路
展开
把直线的 y 表达式代入曲线方程。先求交点的横坐标,再用直线求纵坐标。
答题过程
展开
Here y=11x+3, so
2x2−2x(11x+3)+2=0⟹10x2+3x−1=(5x−1)(2x+1)=0.
The roots are x=51 and x=−21. Substitution into the line gives the intersections (51,526) and (−21,−25).
(b)
解法一
思路
展开
代入直线后,若二次方程没有实根就没有交点。另检查二次项系数为零时的退化情形。
答题过程
展开
With y=4x+3, intersections correspond to
(2−4k)x2−3kx+2=0.
If k=21, this becomes −23x+2=0, which has a solution. For k=21, there are no real roots precisely when the discriminant is negative:
(−3k)2−4(2−4k)(2)=9k2+32k−16=(k+4)(9k−4)<0.
Thus −4<k<94, which already excludes k=21.