(sinx)^2=1/2-1/2cos2x则原级数=(1-cos2x)/2=1/2[1-1+(2x)²/2!-(2x)^4/4!+...]=1/2[(2x)²/2!-(2x)^4/4!+...]=x²-2^3x^4/4!+2^5x^6/6!-.....
