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a1=0 a2=6=a1+6 a3=-6=a2-12 a4=18=a3+24 a5=-30=a4-48 a6=66=a5+96 …… an=a(n-1)-3*(-2)^(n-1) 各式相加得 an=0+6-12+24-……-3*(-2)^(n-1) =6*[1-(-2)^(n-1)]/[1-(-2)] =2*[1-(-2)^(n-1)] =(-2)^n+2
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