Insert 5 numbers between 19 and 91 to form an arithmetic sequence. The sum of these 7 numbers is______ ．

(19 + 91) × 7 / 2, = 110 × 7 / 2, = 385; so the answer is: 385

Write the general formula of the following numbers: 3,1,6,4,9,7,12,10,15,13,18,16,21,19,

An = 3 + 3 (n-1) / 2 n is odd, an = 1 + [(n-2) / 2] X3 n is even
Thank you!

Sequence - 1, 7, - 13, 19 The general formula of is______ ．

∵1，7，13，19，… The general formula of this sequence is 1 + (n-1) × 6 = 6n-5, sequence-1,7, - 13,19 The general formula of is (- 1) n (6n-5)

How to express the nth of sequence 1, 7, 19, 37, 61?

a(n+1)=a(n)+6*n
a(n)=1+6*(1+2+...n-1)=1+6*(n-1)n/2=3n^2-3n+1

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