ab=1a=n²+3n+2b=1/(n²+3n+2)b=1/[(n+2)(n+1)]b=1/(n+1) - 1/(n+2)b1加到b10=1/2-1/3+1/3-1/4+...+1/11 - 1/12 =1/2-1/12 =5/12