運算式 18: "R" Subscript, 2 , Baseline left parenthesis, "n" , right parenthesis equals 1 plus StartFraction, "n" left parenthesis, "n" minus 3 , right parenthesis Over 2 , EndFraction plus left parenthesis, StartFraction, "n" Over 2 , EndFraction minus 1 , right parenthesis "d" left parenthesis, 2 , "n" , right parenthesis plus "a" Subscript, 2 , Baseline left parenthesis, "n" , right parenthesis plus 2 "a" Subscript, 3 , Baseline left parenthesis, "n" , right parenthesis plus 3 "a" Subscript, 4 , Baseline left parenthesis, "n" , right parenthesis plus 4 "a" Subscript, 5 , Baseline left parenthesis, "n" , right parenthesis plus 5 "a" Subscript, 6 , Baseline left parenthesis, "n" , right parenthesis plus 6 "a" Subscript, 7 , Baseline left parenthesis, "n" , right parenthesisR2n=1+nn−32+n2−1d2,n+a2n+2a3n+3a4n+4a5n+5a6n+6a7n