Quay lại đáp án

	2	thêm 16 ký tự trong đáp án		Sửa 04-03-16 08:05 PM Confusion 40 7
ta có $sin2A+sin2B+sin2C$ $=$$2sin(A+B)cos(A-B) + 2 sinCcosC$ $=2sinCcos(A-B)+2sinCcosC$ $=2sinC ( cos(A-B) + cosC)$ $=2sinC ( cos(A-B) - cos(A+B))$ $=2sinC.2sinAsinB$ $=4sinAsinBsinC$ ta có sin2A+sin2B+sin2C =2sin(A+B)cos(A-B) + 2 sinCcosC =2sinCcos(A-B)+2sinCcosC =2sinC ( cos(A-B) + cosC) =2sinC ( cos(A-B) - cos(A+B)) =2sinC.2sinAsinB =4sinAsinBsinC ta có $sin2A+sin2B+sin2C$ $=$$2sin(A+B)cos(A-B) + 2 sinCcosC$ $=2sinCcos(A-B)+2sinCcosC$ $=2sinC ( cos(A-B) + cosC)$ $=2sinC ( cos(A-B) - cos(A+B))$ $=2sinC.2sinAsinB$ $=4sinAsinBsinC$

	1	tạo mới		Trả lời 04-03-16 07:57 PM @_@ Mèo9119 @_@ 1
ta có sin2A+sin2B+sin2C =2sin(A+B)cos(A-B) + 2 sinCcosC =2sinCcos(A-B)+2sinCcosC =2sinC ( cos(A-B) + cosC) =2sinC ( cos(A-B) - cos(A+B)) =2sinC.2sinAsinB =4sinAsinBsinC

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