Rút gọn các tổng sau: S1=14cos2a2+ 142cos2a22+...+14ncos2a2nS2=1sinx.sin2x+1sin2x.sin3x+...+1sinnx.sin(n+1)xS3=(1+1cosx)(1+1cos2x)...(1+1cos2n−1x)S4=1sinx+1sin2x+...+1sin2n−1xS5=(2cosx−1)(2cos2x−1)...(2cos2n−1x−1)S6=1cosa+cosa+1cosa+cos3a+...1cosa+cos(2n+1)a