How to write the fourth question, high school math, ask for help. - Training Enrollment Network

How to write the fourth question, high school math, ask for help.

Sn=n？ - 9n

So S(n- 1)=(n- 1)? - 9(n- 1)

Subtract two expressions: an=n? - 9n - [(n- 1)？ - 9(n- 1)]

∴an=2n - 10

∵5 & lt； Ak< ba

∴5<； 2k- 10 & lt； eight

15 & lt； 2k< 18, then 7.5.

∴k=8

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