tính sin18: sin 18=sin( /5)
ta có:sin(2 /5)=sin(3 /5)
2sin( /5)cos( /5)=3sin( /5)-4sin^3( /5)
sin( /5)[2cos( /5)-3+4sin^2 ( /5)]=0
sin( /5)[2cos( /5)-3+4-4cos^2( /5)]=0
sin( /5)[4cos^2( /5)-2cos( /5)-1]=0
sin( /5)=0 (loại)
cos( /5)=-0,319016994 (loại)
cos( /5)=0,809016994=(1+ :sqrt{5})/4
sin^2( /5)=1-cos^2( /5)
sin18
xuanquynh
xuanquynh
Member Since 13-02-2005Offline Last Active 26-04-2008 - 12:04