pt lượng giác

M

mua_sao_bang_98

1. $cosxcos2xcos4xcos8x=\frac{1}{16}$

\Leftrightarrow $sinx.cosxcos2xcos4xcos8x=\frac{1}{16}sinx$

\Leftrightarrow $\frac{1}{2}sin2xcos2xcos4xcos8x=\frac{1}{16}sinx$

\Leftrightarrow $sin4xcos4xcos8x=\frac{1}{4}sinx$

\Leftrightarrow $sin8xcos8x=\frac{1}{2}sinx$

\Leftrightarrow $sin16x=sinx$

Đến đây dễ rồi
 
M

mua_sao_bang_98

2. $4sin3xcos2x=1+6sinx-8sin^3x$

\Leftrightarrow $4sinx(3-4sin^2x)cos2x=1+6sinx-8sin^3x$

\Leftrightarrow $12sinxcos2x-16sin^3xcos2x-1-6sinx+8sin^3x=0$

\Leftrightarrow $12sinx(1-2sin^2x)-16sin^3x(1-2sin^2x)-1-6sinx+8sin^3x=0$

\Leftrightarrow $12sinx-24sin^3x-16sin^3x+32sin^5x-1-6sinx+8sin^3x=0$

\Leftrightarrow $32sin^5x-32sin^3x+6sinx-1=0$

Thử giải pt ẩn sinx thử xem sao
 
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