Cho góc $xOz=120^0$, Oy là tia phân giác của góc xOz, Ot là tia phân giác của góc xOy, M là điểm thuộc miền trong của góc yOz. Vẽ MA vuông góc Ox, vẽ MBvuông góc Oy, vẽ MC vuông gócOt. Tính độ dài OC theo MA và MB
bài này quen nhỉ –  congiola_ktqd 19-11-12 08:54 PM

Đặt: $\angle BOM=\alpha$
Suy ra: $MA=MO\sin(\frac{\pi}{3}+\alpha)$
             $MB=MO\sin\alpha$
             $OC=MO\cos(\frac{\pi}{6}+\alpha)$
Ta có:
$MA-MB=MO(\sin(\frac{\pi}{3}+\alpha)-\sin\alpha)$
                       $=2MO\cos(\frac{\pi}{6}+\alpha)\sin\frac{\pi}{6}$
                       $=MO\cos(\frac{\pi}{6}+\alpha)=OC$
Suy ra: $OC=MA-MB$
lời giải hay! –  congiola_ktqd 19-11-12 08:55 PM

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