nilai dari 2 sin (1/4phi + x) sin (1/4phi - x) adalah

2sin(1/4π+x)sin(1/4π-x)
= -{cos[(1/4π+x)+(1/4π-x)] - cos[(1/4π+x)-(1/4π-x)]}
= -{cos 1/2π - cos2x}
= -{cos90 -cos 2x}
= 0 + cos 2x
= cos2x