Bài 9:
a) `cos^2 \alpha-sin^2 \alpha = 1- 2sin^2 \alpha`
`VT= cos^2 \alpha-sin^2 \alpha`
`= (1-sin^2 \alpha)-sin^2 \alpha`
`=1-2sin^2 \alpha = VP` (ĐPCM)
b) `2cos^2 \alpha-1=1-2sin^2 \alpha`
`VT= 2cos^2 \alpha-1`
`= 2(1-sin^2 \alpha)-1`
`=1-2sin^2 \alpha = VP` (ĐPCM)
Bài 10:
a) `tan x+cot x=\frac{1}{sin x.cos x}`
`VT= tan x +cot x`
`= \frac{sin x}{cos x}+\frac{cos x}{sin x}`
`=\frac{sin^2x+cos^2x}{sinxcosx}`
`=\frac{1}{sinxcosx}=VP` (ĐPCM)
b) `\frac{1-cos x}{sin x}=\frac{sin x}{1+cos x}`
`VT= \frac{1-cos x}{sin x}`
`=\frac{1-cos^2x}{sinx.(1+cosx)}`
`=\frac{sin^2x}{sinx.(1+cosx)}`
`=\frac{sinx}{1+cosx}=VP` (ĐPCM)
