一、当O在ABCD内部时,过O作AB的垂线交AB、CD于E、F,再过O作BC的垂线交BC、AD于G、H.OA^2＝AH^2＋OH^2＝OE^2＋OH^2,　OC^2＝OG^2＋GC^2＝OG^2＋OF^2∴OA^2＋OC^2＝OE^2＋OH^2＋OG^2＋OF^2······①OB^2＝BG^2＋O...