Gợi ý vài câu lượng giác với?

a)

cos^2x.(sin^2x -10 sinx.cosx +21cos^2x)=0

=>

[cosx =0

[sin^2x -10 sinx.cosx +21cos^2x=0 (chia 2 vế ch cos^2x.. giai ptb2 an tg x)

b)

(sinx -cosx)(sin^4x +sin^3x.cosx + sin^2x.cos^2x + sinx.cos^3x +cos^4x) = (sinx-cosx)\(sinx.cosx)

<=>

[sinx-cosx =0

[sin^4x +sin^3x.cosx + sin^2x.cos^2x + sinx.cos^3x +cos^4x = 1\(sinx.cosx) (*)

(*) <=> (sin^4x +2sin^2x .cos^2x +cos^4x) +sinx.cosx.(sin^2x +cos^2x) - sin^2x.cos^2x = 1\sinx.cosx

<=> 1 +sinx.cosx - sin^2x.cos^2x = 1\sinx.cosx

<=> 1 + a -a^2 =1\a

=> a^3 -a^2 -a +1=0

=> a= sinx.cosx = (sin2x)\2 =.. => x=...

cau a) nhu vp

cau b) sin^5 x-cos^5 x=(sinx-cosx)(sin^4x+cosxsin^3x+

+cos^2xsin^2x+sinxcos^3x+cos^4x)

ban giai 2

PT 1) cosx=sinx

2) dat t=sinxcosx=>(t-1)^2(t+1)=0