抛物线y^2=4x截直线y=2x+k所得弦长为3根号5,则k的值如题
问题描述：
抛物线y^2=4x截直线y=2x+k所得弦长为3根号5,则k的值
如题
答
x=(y-k)/2 y^2-2(y-k)=0 y1+y2=2 y1*y2=2k 弦长平方=（x1-x2)^2+(y1-y2)^2x1-x2=(y1-y2)/2 弦长平方=（x1-x2)^2+(y1-y2)^2=(y1-y2)^2/4+(y1-y2)^2=(5/4)(y1-y2)^2=5/4[(y1+y2)^2-4y1y2]=5/4[4-8k]=[3根号5]^2=45 4-8...