6ec8aac122bd4f6e=(3×2+2)+ (3×6ec8aac122bd4f6e+2)+ (3×6ec8aac122bd4f6e+2)+……+(3×6ec8aac122bd4f6e+2)   =3・6ec8aac122bd4f6e+2n=7・6ec8aac122bd4f6e-6.(分组求和法)

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