Chứng Minh: 
a)  $\frac{ sinx + sin2x + sin3x }{ cosx + cos2x + cos3x } = tan2x$
b) $cosx.cos (Π/3 - x) .cos(Π/3 + x) = 1/4 cos3x$
c)$ sin5x - 2sinx.( cos4x + cos2x) = sinx$
Có TT giúp mình đi –  hgdam25 05-04-16 09:32 PM
cần mình làm câu c) k –  nguyenquangtruonghktcute 05-04-16 09:15 PM
Mình vừa sửa r đó bạn –  hgdam25 05-04-16 09:06 PM
câu c) đề đúng k bạn –  nguyenquangtruonghktcute 05-04-16 09:04 PM
b) $cosx.cos(\pi /3-x).cos(\pi /3+x)$
$=cosx.\frac{1}{2}(cos2x+cos\frac{2\pi }{3})$
$=cosx.\frac{1}{2}(2cos^2x-\frac{3}{2})$
$=cos^3x-\frac{3}{4}cosx=\frac{1}{4}cos3x$

Cần trả +1,000vỏ sò để xem nội dung lời giải này

Còn phần b và c T.T hộ e luôn đi –  hgdam25 05-04-16 08:29 PM
c)..............
$sin5x-2sinx.cos4x-2sinx.cos2x$
$sin5x-sin5x+sin3x-sin3x+sinx=sinx$
ko hiểu chỗ lsao ra đc sin5x sin3x - sin3x sinx ấy –  hgdam25 05-04-16 09:50 PM
hơi tắt. k hiểu chỗ nào nói nhag :v –  nguyenquangtruonghktcute 05-04-16 09:45 PM
$2sinx.cos4x=sin(x+4x)+sin(x-4x)$
$2sinx.cos2x=sin(x+2x)+sin(x-2x)$
dựa theo công thức có trong sách giáo khoa lớp 10 á
À đc r. =)) cám ơn bạn –  hgdam25 05-04-16 09:58 PM

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