cos3x=2sin( 3pi/2-5x)

0

01697981687

ta có cos3x=2sin(3pi/2-5x)
<=>cos3x=-2cos5x
nhân 2 vế vs 2cosx
<=>2cosxcos3x=-4cosxcos5x
<=>cos4x+cos2x=-2(cos4x+cos6x)
<=>3(2cos^{2}2x-1)+cos2x+2(4cos^{3}2x-3cos2x)=0
<=>8cos^{3}2x+6cos^{2}2x-5cos2x-3=0
đến đây bạn tự giải
 
You must log in or register to reply here.
Top Bottom