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Precalculus

Natural logs and the number e

Use ln and eˣ, the natural log and its inverse, based on e ≈ 2.71828.

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The number e is about 2.718281828. The natural log ln(x) asks: what power of e gives x? The key eˣ does the opposite.

ln(20) = 2.995732274, and e raised to that gives 20 again.

Classic Graphing 84

Start on the main screen: Press 2nd then mode (QUIT) to get back to the home screen. Pressing clear on a menu also backs out of it.

  1. Press ln, type 20, close the bracket and press enter.

    ln20)enter

  2. Press 2nd then ln for eˣ, then 2nd and (−) for Ans. Close the bracket and press enter.

    2ndeˣln2ndANS(−))enter

You should seeln(20) 2.995732274 e^(Ans) 20

Classic Graphing 84 key reference

Classic Scientific 30

Start on the main screen: Press 2nd then mode (QUIT) to leave any menu or screen and return to the calculation screen.

  1. Press ln, type 20, close the bracket and press enter.

    ln20)enter

  2. Press 2nd then ln for eˣ, then 2nd and (−) for Ans, and press enter.

    2ndeˣln2ndans(−)enter

You should seee^(Ans) 20

Classic Scientific 30 key reference

Natural Scientific 991

Start on the main screen: Press MENU, then 1 (Calculate) to return to the main calculation screen. AC clears the line you are on.

  1. Press ln, type 20 and press =.

    ln20=

  2. Press SHIFT then ln for eˣ, then Ans, and press =.

    SHIFTelnAns=

You should seee^(Ans) 20

Natural Scientific 991 key reference

Natural Color Graphing

Start on the main screen: Press MENU then 1 to open Run-Matrix, the main calculation screen. Press EXIT to back out of a bar or screen, and SHIFT then EXIT (QUIT) to return to the app start.

  1. Press ln, type 20 and press EXE.

    ln20EXE

  2. Press SHIFT then ln for eˣ, then SHIFT and (−) for Ans, and press EXE.

    SHIFTeˣlnSHIFTAns(−)EXE

You should see2.995732274 e^(Ans) 20

Natural Color Graphing key reference

Classic CAS

Start on the main screen: Press home to open the Home menu (Calculator, Graphs, Settings, and more). Choose Calculator, or press esc to go back to the calculation page.

  1. Type ln(20) and press ctrl enter for a decimal.

    ln20)ctrl≈enter

  2. Press ctrl then ln for eˣ, then ctrl and (−) for ans, and ctrl enter.

    ctrleˣlnctrlans(−)ctrl≈enter

You should see2.99573 e^(ans) 20.

Classic CAS key reference

RPN Scientific 15

Start on the main screen: There is no menu to leave. Press the back arrow to clear a number you are typing, or CLx (g then back arrow) to clear the X register.

  1. Key in 20. Press g then the eˣ key to use LN.

    20gLNeˣ

  2. Press the eˣ key (no prefix) to undo it.

    eˣ

You should see20.0000

RPN Scientific 15 key reference

RPN Advanced 42

Start on the main screen: Press EXIT (once or twice) to close any menu and return to the plain stack view of Y and X.

  1. Key in 20 and press LN, then shift and LN for eˣ.

    20LN⇧eˣLN

You should seex: 20.0000

RPN Advanced 42 key reference

RPN Power 48

Start on the main screen: Press ON to cancel what you are typing, then use the stack view. Open the MODE or VAR menu any time; the stack is always shown above the menu row.

  1. Key in 20 and press the right shift and yˣ for LN.

    20▶LNyˣ

You should see2.99573227355

Left shift and yˣ gives eˣ.

RPN Power 48 key reference

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