級数
1/1-1/3+1/5-1/7+・・・=π/4
===================================
[4]sinx=1,すなわち,
1-x/1+x^3/1・2・3-x^5/1・2・3・4・5+・・・=0
の根はx=±π/2,±3π/2,±5π/2,・・・
[5]1-sinx=(1-2x/π)^2(1+2x/3π)^2(1-2x/5π)^2・・・
-1=(-4/π+4/3π-4/5π+・・・)
[6]1/1-1/3+1/5-1/7+・・・=π/4
===================================