假设y^=2px,把y=2x+1带入,
4x^-(4+2p)x+1=0,
x1+x2=(4+2p)/4,
x1x2=1/4,
｜x1-x2｜=√〔(4+2p)^/16 -1〕,
弦长^2=15=（1+k^)｜x1-x2｜=5〔(4+2p)^/16 -1〕,
解得：p=2,-6;
抛物线的方程:y^=4x 或者y^=-12x.