S(ABCD) = S(AOB) +S(BOC) +S(COD) +S(AOD) =
1/2 OA*OB + 1/2 OB*OC + 1/2 OC*OD + 1/2 OA*OD =
1/2 (OB(OA+OC) + OD(OA+OC)) =
1/2 (OB+OD)(OA+OC) =
1/2 BD*AC
S(ABCD) =1/2 *4*10 =20 (см^2)
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S(ABCD) = S(AOB) +S(BOC) +S(COD) +S(AOD) =
1/2 OA*OB + 1/2 OB*OC + 1/2 OC*OD + 1/2 OA*OD =
1/2 (OB(OA+OC) + OD(OA+OC)) =
1/2 (OB+OD)(OA+OC) =
1/2 BD*AC
S(ABCD) =1/2 *4*10 =20 (см^2)