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AE = (1/2)[AB + AC]
CF = (1/2)[CA + CD]
= (1/2)[-AC + AD - AC]
= (1/2)AD - AC
AB * AD = 36 / 2 = 18
= AC * AD
= AB * AC
AC * AC = 36
AE * CF = (1/2)[9 - 18 + 9 - 36]
= (1/2)[-36]
= -18
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