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f(x)=1-2((1/2)sin2x)²
f(x)=1-(1/2)(sin2x)²
sin2x∈<-1,1>
(sin2x)²∈<0,1>
(1/2)(sin2x)²∈<0,1/2>
-(1/2)(sin2x)²∈<-1/2,0>
1-(1/2)(sin2x)²∈<1/2,1>
2.
sin50+sin10-cos20=2sin[(50+10)/2]cos[(50-10)/2]-cos20=2sin30cos20-cos20=2*(1/2)cos20-cos20=cos20-cos20=0