Hình 8

T

thetimeforus

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T

trinhminh18

a/ Hình bình hành:
b/ $S_{EFGH}= S_{ABCD}-2(S_{AEH}+S_{EBF})=16.8-2(\dfrac{a(8-a)}{2}+\dfrac{a(16-a)}{2})=...$
c/$S_{EFGH} min$ khi $(S_{AEH}+S_{EBF}) max$
\Leftrightarrow $\dfrac{a(8-a)}{2}+\dfrac{a(16-a)}{2} max$
Ta có: $\dfrac{a(8-a)}{2}+\dfrac{a(16-a)}{2} = \dfrac{24a-2a^2}{2} =12a-a^2 = 36- (a-6)^2 $ \leq 36
$Max (S_{AEH}+S_{EBF}) =36 $ khi $a=6$
 
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