an = a1+(n-1)d
其中奇數(shù)項(xiàng)的和為90
a1+a3+..+a(2n-1) =90
(a1+(n-1)d)n =90(1)
偶數(shù)項(xiàng)和是72
a2+a4+..+a2n=72
(a1+nd)n = 72(2)
(2) -(1)
nd=72-90
等差數(shù)列{an}共有2n項(xiàng),其中奇數(shù)項(xiàng)的和為90,偶數(shù)項(xiàng)和是72,且a2n-a1=-33,那么它的公差為?
等差數(shù)列{an}共有2n項(xiàng),其中奇數(shù)項(xiàng)的和為90,偶數(shù)項(xiàng)和是72,且a2n-a1=-33,那么它的公差為?
還有啊,為什么nd=90-72?
還有啊,為什么nd=90-72?
數(shù)學(xué)人氣：166 ℃時(shí)間：2020-03-25 19:13:41
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