In the positive proportional sequence {an}, A4 + a3-a2-a1 = 5, then the minimum value of A5 + A6 is______ ．

In the positive proportional sequence {an}, A4 + a3-a2-a1 = 5, then the minimum value of A5 + A6 is______ ．

In the positive term sequence {an}, let A2 + A1 = x, and the common ratio of the sequence is Q, then A4 + a3 = Xq2, A5 + A6 = XQ4. Then from A4 + a3-a2-a1 = 5, we can get Xq2 = 5 + X, ｜ x = 5q2 − 1 ＞ 0, Q ＞ 1; If and only if q2-1 = 1, the equal sign holds, so the minimum value of A5 + A6 is 20, so the answer is 20

If a1 + A2 = 1, A3 + A4 = 4, then A5 + A6=

A1 + A2 = 1, A3 + A4 = 4, then A5 + A6=
Because it is an equal ratio sequence, let the common ratio be n, then A2 = n, A1, A3 = n & sup2; A1, A4 = n & sup3; A1
Then we put forward n & sup2; in those two formulas and get n & sup2; = 4
Then A5 + A6 = n quartic * (a1 + A2) = 16

Given a series of numbers A1, a2a3a4a5a6a7 and A1 = 1, a7 = 729, A1 / A2 = A2 / A3 = A3 / A4 = A4 / A5 = A5 / A6 = A6 / A7, find out the number

From A1 / A2 = A2 / A3 = A3 / A4 = A4 / A5 = A5 / A6 = A6 / A7, we can see that this is an equal ratio sequence
The formula an = A1 * q ^ (n-1)
Substituting A1 = 1 A7 = 729 into formula 729 = 1 * q ^ (7-1)
Equality q = 3
And then we can figure it out
a1=1
a2=3
a3=9
a4=27
a5=81
a6=243
a7=729

If A1 = 3, A2 = 9, A3 = 27, A4 = 81, A5 = 243, A6 = 729, a7 = 2187, a8 = 6561... Then the number of a2013 is?
A.3 B.9 C.7 D.1

Single digit rule: 3,9,7,1 cycle
2013=4*503+1
Its single digit is the single digit 3 of the first item