当k为何值时,直线y=kx+k-2与抛物线y2=4x有两个公共点,仅有一个公共点

当k为何值时,直线y=kx+k-2与抛物线y2=4x有两个公共点,仅有一个公共点,无公共点?

答案

思路解析:可以考虑数形结合,也可以看方程组解的个数.

解:由方程组消去y,得k2x2+2(k2-2k-2)+(k-2)2=0.

(1)当k=0时,直线y=-2与抛物线y2=4x仅有一个公共点.

(2)当k≠0时,Δ=4(k2-2k-2)2-4k2(k-2)2=-16(k2-2k-1).

当k∈(1-,0)∪(0,1+)时,Δ>0,直线与抛物线有两个公共点;

当k=1±时,Δ=0,直线与抛物线仅有一个公共点;

当k∈(-∞,1-)∪(1+,+∞)时,Δ<0,直线与抛物线没有公共点.

综合以上情况,当k∈(1-,0)∪(0,1+)时,直线与抛物线有两个公共点;当k=0或k=1±时,直线与抛物线仅有一个公共点;当k∈(-∞,1-)∪(1+,+∞)时,直线与抛物线没有公共点.

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