toán về lượng giác

M

maingockien2506

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K

kisihoangtoc

cosA= AE/AB=AF/AC
=> Cos^2A= AE.AF/AB.AC=SAFE/SABC
tuong tu
cos^2B= SDBF/SABC
Cos^2C= SDEC/SABC
Ma SAFE/SABC + SDBF/SABC + SDEC/SABC + SDEF/SABC =1
<=> Cos^2A+cos^2B+Cos^2C+SDEF/SABC=1
 
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