When used in the negative, てくれない (coming from てくれる), and てもらえない (coming from てもらえる), are similar to the negative question form of じゃないか. They express the following:
てくれない - Would you not do (A) for me?
てもらえない - Could I not have you do (A) for me?

• 私（わたし）に漢字（かんじ）を教（おし）えてくれない？
  Would you not teach me kanji? (Would you please teach me kanji?)
• 道（みち）を調（しら）べてもらえない？
  Could I get you to look up the way?

In both of these constructions. か (the question particle) may be omitted. However, they do sound slightly more polite when か is used.

• 今日（きょう）だけ自転車（じてんしゃ）を貸（か）してくれないか？
  Just for today, would you not let me borrow your bike?
• テレビを点（つ）けてもらえないか。
  Could I get you to turn the TV on for me, please?

Let’s examine the following list, for a scale of the level of politeness.
てくれ, てくれる, てもらえる, てくれない（か）, てもらえない（か）