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Contemporary middle school students apply for the answer to the fourth question of compulsory mathematics 5.
Ability improvement

volume one

CDBAB DDACB

(-1+ radical number 5)/2 5 9 6

q=2 n=8

t=( 1/7)*(8^(n+ 1)- 1)

Volume II

CCCB

((-1+ radical number 5)/2, (1+ radical number 5)/2) 4 or 5/2.

an=2^n t=2+(n- 1)*2^(n+ 1)

an=2^(n- 1)s=(4^(n)- 1)/3+(n^(2)-n)/2

There is no process, dear ~ do it yourself! For reference, the sixth issue gives the answer. ...

The first line is a single choice.

The second line is to fill in the blanks

The third line is the first big question.

The fourth line is the second big problem.

Ps: I can't type mathematical symbols, so I'm just looking…… around ~ (pointing to my ing ..............)