| 与不等式の左辺はx=πに関して対称なので、0<x≦πに関して示せば十分。
0<x≦π/2のとき sinx>x-x^3/6=(-x^3+6x)/6 cosx<1-x^2/2+x^4/24=(x^4-12x^2+24)/24 2sin(x/2)<xから log(2sin(x/2))<logx<(x-1)-(x-1)^2/2+(x-1)^3/3 =(2x^3-9x^2+18x-11)/6 なので (π-x)sinx-2(cosx)log(2sin(x/2)) >(3-x)(-x^3+6x)/6-2(x^4-12x^2+24)/24・(2x^3-9x^2+18x-11)/6 =(-2x^7+9x^6+6x^5-85x^4+132x^3+12x^2-216x+264)/72 =(2t^7+129xt^5+(783x^3+4796)t^2+3(5x+36)t^3+17654t+41984)/157464+1 >1 (ただしt=5-3x>0)
π/2≦x≦πのときy=π-xとおくと0≦y≦π/2で (π-x)sinx-2(cosx)log(2sin(x/2)) =ysin(π-y)-2(cos(π-y))log(2sin((π-y)/2)) =ysiny+2(cosy)log(2cos(y/2)) siny>y-y^3/6=(-y^3+6y)/6 cosy>1-y^2/2=(-y^2+2)/2 2cos(y/2)>(16-3y)/8から log(2cos(y/2))>log((16-3y)/8)>(8-3y)/8-((8-3y)/8)^2/2 =(-9y^2+64)/128 なので ysiny+2(cosy)log(2cos(y/2)) >y(-y^3+6y)/6+2(-y^2+2)/2・(-9y^2+64)/128 =(-37y^4+138y^2+384)/384 =y^2(138-37y^2)/384+1 ≧1
∴(π-x)sinx-2(cosx)log(2sin(x/2))≧1
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