tra loi bai tap" mik nghi cau tra loi nhu the, ko bit co dung ko nua"
A=sinx+sin(x+a)+sin(x+2a)+sin(x+3a)+sin(x+4a)
=[sinx+sin(x+4a)]+[sin(x+a)+sin(x+3a)]+sin(x+2a)
=2sin(x+2a)cos2a+2sin(x+2a)cosa+sin(x+2a)
=sin(x+2a)[2cos2a+2cosa+1]
Để A ko phụ thuộc vào x thì sin(x+2a) phải = 0 hoặc 1
* vs sin(x+2a)=0\Rightarrowx+2a=k\prod_{i=1}^{n}
\Rightarrowx=-2a+k\prod_{i=1}^{n}
\Rightarrowk\prod_{i=1}^{n}<x<-2\prod_{i=1}^{n}+k\prod_{i=1}^{n}
*vs sin(x+2a)=1\Rightarrowx+2a=\prod_{i=1}^{n}/2+k2\prod_{i=1}^{n}
\Rightarrowx=-2a+\prod_{i=1}^{n}/2+k2\prod_{i=1}^{n}
\Rightarrow\prod_{i=1}^{n}/2+k2\prod_{i=1}^{n}<x<-3/2\prod_{i=1}^{n}+k2\prod_{i=1}^{n}
Mik nghi de la tim x, p xem lai nha