题干

阅读材料:

如图(1),在四边形ABCD中,对角线AC⊥BD,垂足为点P.求证:S四边形ABCD=

1
2
AC•BD;

证明:∵AC⊥BD,

{SACD=

1
2
ACPDSABC=
1
2
ACBP

∴S四边形ABCD=SACD+SACB=

1
2
AC•PD+
1
2
AC•BP

=

1
2
AC(PD+PB)=
1
2
AC•BD

解答问题:

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