Dawnbridge

TI-Nspire CX II field manual for IB Math

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26

Inverse normal

All courses

Use the inverse-normal routine to find the value k with a given cumulative probability P(X<k).

Goal

If ​X∼N(60,52), find ​k​ such that ​P(X<k)=0.9.

1

From the Scratchpad calculator, press → 6: Statistics → 5: Distributions → 3: Inverse Normal.

2

Fill in the dialog, moving between fields with : Area: 0.9 (the probability to the left of ​k​), ​μ=60,  ​σ=5.

Scratchpad
RAD
Inverse Normal
Area:0.9▶
μ:60▶
σ:5▶
OKCancel
3

Select OK and press . The invNorm( command lands on the calculator page with its value.

Result

​k≈66.4.

Scratchpad
RAD
invNorm(0.9,60,5)
66.4078

Your turn

Work each one on your calculator, then check the answer.

  1. 1

    If ​X∼N(100,152), find ​k​ such that ​P(X<k)=0.95.

  2. 2

    If ​X∼N(24,32), find ​k​ such that ​P(X<k)=0.10.

  3. 3

    If ​X∼N(70,62), find ​k​ such that ​P(X<k)=0.25.