Bai luong giac kho....

V

vovandong

ta co cotx^2=1/(1-cosx^2)-1
dat t=cosx,dk of t
co 2can2t^4+(2+3can2)t^3+(3-4can2)t^2-(2+3can2)t+2can2=0
giai dc nghiem la t=can2/2;t=-2;t=-can2;t=1/2 ket hop dk of t suy ra nghiem
 
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