[Toán 10] Lượng giác

L

lp_qt

Câu 2

$ sina.\cos ^5 a - \sin^5 a. cosa =\dfrac{1}{2}.\sin2a.\cos^4a-\dfrac{1}{2}.\sin2a.\sin^4a$

$=\dfrac{1}{2}.\sin2a(\cos^4a-\sin^4a)=\dfrac{1}{2}.\sin2a(\cos^2a-\sin^2a)(\cos^2a+\sin^2a)=\dfrac{1}{2}.\sin2a.\cos2a=\dfrac{1}{4}.\sin4a$
 
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D

dien0709

$(2sin10^o+1).cos50^o=2cos50^osin10^o+cos50^o$

$=sin60^o-sin40^o+sin40^o=sin60^o=\sqrt{3}/2$
 
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