證明:
四邊形ABCD對(duì)角線AC⊥BD,AC和BD相交于點(diǎn)O
S四邊形=S△ADB+S△CDB
=BD×AO÷2+BD×CO÷2
=BD×(AO+CO)÷2
=BD×AC÷2
=對(duì)角線乘積的一半
命題得證