mo1 13天以前 参考・概略です (1) Σ(3k²-k) =3ΣK²-ΣK =3・(1/6)n(n+1)(2n+1)-(1/2)n(n+1) =(1/2)n(n+1){(2n+1)-1} =(1/2)n(n+1){2n} =n²(n+1) (2) Σ(k²+k) =ΣK²+ΣK =(1/6)n(n+1)(2n+1)+(1/2)n(n+1) =(1/6)n(n+1){(2n+1)+3} =(1/6)n(n+1){2n+4} =(1/3)n(n+1)(n+2) 留言