Tính cos24+cos48 -cos84-cos12?
is 2 2 24s of of of oie oie of 2s fs of 3 2 3E 3E 3E 3É ois ok 2s ok
Trước tiên cần tính sin 18
Ta có cos 36 = sin (90-36) = sin 54
<=> | - 2*(sin18)’2 = 3sin18 - 4 (sin18)*3
<=>4(sin18)*3 - 2(sin18)°2-3sinx+1=0
<=> sin 18 =( /5 - 1)/4=t (can bac hai cua 5 trir 1 chia 4)
=> cos 36= | - 2 (sin 18) “2 = 1 - 242 =(1+ /5)/4
is 2 2s of of of of oie of of fs os of of 2 2s 2k 2k 2k ok ok ok
A=cos 24 + cos 48- cos84 - cos 12
= [cos24+cos48] -[cos84+cos 12]
= 2*cos36*cos12 -2*cos48*cos36
= 2*cos36(cos12-cos48)
= 2*cos36* 2*sin30*sin18
= 2 *sinl8*cos36
=2*#(./5-1/4*(/5+1/⁄4
=1/2
Vay cos24+cos48 -cos84-cos12 = 1/2