Add missing chapter 9 exercises

[MarcoGorelli]

The last few exercises from Chapter 9 were missing, so I figured I'd add them in

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MarcoGorelli commented on 2021-07-19T20:48:13Z ----------------------------------------------------------------

this isn't very precise, I'll fix it up later

MarcoGorelli commented on 2021-07-20T20:07:02Z ----------------------------------------------------------------

done

[MarcoGorelli]

done

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