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x²项系数:2+ 2*3/2+ 3*4/2+4*5/2+……+30*31/2 = 1+ (1²+1)/2 + (2²+2)/2 + (3²+3)/2 + (4²+4)/2 +……+(30²+30)/2 = 1+(1²+2²+3²+4²+……+30²)/2 + (1+2+3+4+……+30)/2 =1+ 30*(30+1)(2*30+1)/12 + 30(30+1)/4 = 4961