Đáp án:
$A=tan3x$
Giải thích các bước giải:
$A=\frac{sinx+sin3x+sin5x}{cosx+cos3x+cos5x}\\
=\frac{sin3x+(sinx+sin5x)}{cos3x+(cosx+cos5x)}\\
=\frac{sin3x+2sin\frac{x+5x}{2}cox\frac{x-5x}{2}}{cos3x+2cos\frac{x+5x}{2}cos\frac{x-5x}{2}}\\
=\frac{sin3x+2sin3xcox(-2x)}{cos3x+2cos3xcos(-2x)}\\
=\frac{sin3x(1+2cox(-2x))}{cos3x(1+2cos(-2x))}\\
=\frac{sin3x}{cos3x}\\
=tan3x$