bài tập lượng giác

B

baihocquygia

Ta có
cos69=sin21
cos79=sin11
nên
\Rightarrow cos21.cos11+sin21.sin11 - cos10 = cos(21-11)-cos10=0
 
P

pecan2k15

C=sin200.sin310+cos340.cos50

[TEX][/TEX]
C= sin(180+20).sin(360-50) + cos(360-20).cos50
= sin20.sin50 + cos20.cos50
= cos(50-20) = cos30
=[TEX]\frac{\sqrt{3}}{2}[/TEX]
 
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