It is known that FX = two-thirds of cos π xcos two-thirds of x-sin three-thirds of xsin two-thirds of x-2sinxcosx. When x belongs to two-thirds of π, π, the zero point of FX is zero

f(x)=cos(3x/2)cos(x/2)-sin(3x/2)sin(x/2)-2sinxcosx
=cos(3x/2+x/2)-2sinxcosx
=cos2x-sin2x
=√2(√2/2 *cos2x- √2/2 *sin2x)
=√2 sin(π/4-2x)
f(x)=0
sin(π/4-2x)=0
π/4-2x=kπ
x=π/8-kπ/2
When k = - 1, x = 5 π / 8

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