Chứng minh: tam giác ABC có b= a.Cosc + c.Cos.A
kẻ $BH\bot AC$

xét $\Delta AHB$ vuông tại $H$ có

$AH=AB.cosA$

xét $\Delta AHC$ vuông tại $H$ có

$HC=BC.cosC$

$=>AH+HC=AB.cosA+BC.cosC$

$<=>AC=AB.cosA+BC.cosC$

$<=>b=c.cosA+a.cosC(dpcm)$
đúng thì nhấn V và vote up nhé. –  ♥♥♥ Panda Sơkiu Panda Mập ♥♥♥ 07-04-14 08:10 PM

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