Bài số 2 trước nghe em
a) ~O)
[tex]ta co :\frac{1}{cos^{2}x}=1+tan^{2}x VP= 1+ tan^{2}x -1+sin^{2}x=tan^{2}x +sin^{2}x -->VT=VP [/tex] ->dpcm
b) ~O)
[tex]VT=\frac{tan3x}{tanx}=\frac{sin3x.cosx}{sinx.cos3x}=\frac{sin4x+sin2x}{sin4x-sin2x}=\frac{2cos2x+1}{2cos2x-1}[/tex]
VP=[tex]\frac{3-tan^{2}x}{1-3tan^{2}x}=\frac{3cos^{2}x-sin^{2}x}{cos^{2}x-3sin^{2}x} =\frac{cos2x+2cos^{2}x}{cos2x-2sin^{2}x}=\frac{2cos2x+1}{2cos2x-1}[/tex](công thức hạ bậc)
-->VT=VP dpcm
c) ~O)
[tex]VT= \frac{1}{sin^{2}x}(\frac{1}{cos^{2}x}-1)cos^{2}x =\frac{1}{sin^{2}x}(1-cos^{2}x)=\frac{1}{sin^{2}x}.sin^{2}x=1 -->VT=VP dpcm[/tex]
8-x 8-x *-:) *-:)