Students struggled with…

  • Determining whether to use a binomial or geometric distribution to solve a problem.
  • The calculator commands for binomcdf and geometcdf for “greater than” problems.


  • Encourage students to think through how they would solve the problem without a formula!!!! Students get so stuck on formulas that they sometimes don’t think about a much simpler way to solve a problem. Using the general multiplication rule can be much more simple than using a geometric distribution.
  • Have students write out the values that are possible for the problem.  For example, if we are interested in 5 or more successes in 7 trials, write 0 1 2 3 4 5 6 7 successes. Then have students circle the values they want.  Since we want 5 or more, you would circle 5 6 7.  So you have to ask your calculator for 4 or less since binomcdf(n, p, x) is inclusive.  Students often incorrectly use 5 as the x-value in this scenario. By writing out the values, it’s easier for students to decide what value they need to use for a cdf function.

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