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)
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)
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