求数列1,1-3,1-3+9,1-3+9-27,…前n项的和an = (-3)^0+ (-3)^1 +(-3)^2 +...+(-3)^(n-1)= (1/4)[1- (-3)^n]数列1,1-3,1-3+9,1-3+9-27,…前n项的和这部分看不懂=a1+a2+...+an= (1/4) { n - (-3)[ 1- (-3)^n]/4 }=(1/4) [ n + (3/4)(1-(-3)^n ) ]
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![求数列1,1-3,1-3+9,1-3+9-27,…前n项的和an = (-3)^0+ (-3)^1 +(-3)^2 +...+(-3)^(n-1)= (1/4)[1- (-3)^n]数列1,1-3,1-3+9,1-3+9-27,…前n项的和这部分看不懂=a1+a2+...+an= (1/4) { n - (-3)[ 1- (-3)^n]/4 }=(1/4) [ n + (3/4)(1-(-3)^n ) ]](/uploads/image/z/206988-60-8.jpg?t=%E6%B1%82%E6%95%B0%E5%88%971%2C1-3%2C1-3%2B9%2C1-3%2B9-27%2C%E2%80%A6%E5%89%8Dn%E9%A1%B9%E7%9A%84%E5%92%8Can+%3D+%28-3%29%5E0%2B+%28-3%29%5E1+%2B%28-3%29%5E2+%2B...%2B%28-3%29%5E%28n-1%29%3D+%281%2F4%29%5B1-+%28-3%29%5En%5D%E6%95%B0%E5%88%971%2C1-3%2C1-3%2B9%2C1-3%2B9-27%2C%E2%80%A6%E5%89%8Dn%E9%A1%B9%E7%9A%84%E5%92%8C%E8%BF%99%E9%83%A8%E5%88%86%E7%9C%8B%E4%B8%8D%E6%87%82%3Da1%2Ba2%2B...%2Ban%3D+%281%2F4%29+%7B+n+-+%28-3%29%5B+1-+%28-3%29%5En%5D%2F4+%7D%3D%281%2F4%29+%5B+n+%2B+%283%2F4%29%281-%28-3%29%5En+%29+%5D)
求数列1,1-3,1-3+9,1-3+9-27,…前n项的和an = (-3)^0+ (-3)^1 +(-3)^2 +...+(-3)^(n-1)= (1/4)[1- (-3)^n]数列1,1-3,1-3+9,1-3+9-27,…前n项的和这部分看不懂=a1+a2+...+an= (1/4) { n - (-3)[ 1- (-3)^n]/4 }=(1/4) [ n + (3/4)(1-(-3)^n ) ]
求数列1,1-3,1-3+9,1-3+9-27,…前n项的和
an = (-3)^0+ (-3)^1 +(-3)^2 +...+(-3)^(n-1)
= (1/4)[1- (-3)^n]
数列1,1-3,1-3+9,1-3+9-27,…前n项的和
这部分看不懂
=a1+a2+...+an
= (1/4) { n - (-3)[ 1- (-3)^n]/4 }
=(1/4) [ n + (3/4)(1-(-3)^n ) ]
求数列1,1-3,1-3+9,1-3+9-27,…前n项的和an = (-3)^0+ (-3)^1 +(-3)^2 +...+(-3)^(n-1)= (1/4)[1- (-3)^n]数列1,1-3,1-3+9,1-3+9-27,…前n项的和这部分看不懂=a1+a2+...+an= (1/4) { n - (-3)[ 1- (-3)^n]/4 }=(1/4) [ n + (3/4)(1-(-3)^n ) ]
=a1+a2+...+an
= (1/4){n-[(-3)+(-3)^2+(-3)^3+.+(-3)^n]}
=(1/4){n-(-3)[1-(-3)^n]/[1-(-3)]}
=(1/4){n+3*[1-(-3)^n]}/4
=(1/4)[n+3/4(1-(-3)^n)]