過P點(diǎn),Q點(diǎn)作準(zhǔn)線的垂線（即X軸的平行線）垂足為G ,D 已知PQ過焦點(diǎn)F（p/2,0）,準(zhǔn)線方程為x=--p/2 連DP tan角GPD=DG/GP=(yi-y2)/(x1+p/2)
tan角GPM=y1/x1
tan角GPD--tan角GPM==(yi-y2)/(x1+p/2)--y1/x1
=[y1x1--y2x1-y1x1--y1p/2]/(x1+p/2)x1
=[--y2x1--y1p/2]/(x1+p/2)x1=--[y2(myi+p/2)+y1p/2]/(x1+p/2)x1
=--[my1y2+p/2*(y1+y2)]/(x1+p/2)x1
=--[m(--p^2)+p/2(+2pm)]/(x1+p/2)x1=--0/(x1+p/2)x1=0
tan角GPD--tan角GPM=0 tan角GPD=tan角GPM 由于兩角均為銳角,所以 角GPD=角GPM即MP與PD共線 M點(diǎn)與D點(diǎn)重合 MQ∥于X軸
直線方程 y/(x-p/2)=1/m my=x-p/2 拋物線方程y^2=2px=2pmy+p^2 y^2--2pmy--p^2=0
x1=my1+p/2 y1+y2=+2pm y1y2=--p^2