amaths-quadratic

The key is in the last 3 words of the question.... " in this range". The question is not asking what is the value of k for f(x) has one real root for the whole range of x, but only for the range f(x) < g(x). So -7 is not the only answer, the other possible answer is k less than the y- coordinate of point B and k greater than the y-coordinate of point A, that is 2< k < 18.

因為當你f(x)=k既時候,
個root就係絛equation同y=k呢絛線既交點
而且你得1個real root就代表你絛線想同f(x)只交於一點
而你想絛equation同y=k得1個交點既情況就係k=c既y-axis既value
而且你計過c係(1,-7)
所以k就一定要係-7先得...
你maths書應該教過點樣係個圖度解出quad.equation+linear equation既聯立方程