How can i calculate log inverse with my fx-991ES plus

The inverse of log functions are power functions.
Inv of ln is exponential (e^x)
Inv of log in base 10 is 10^(x)
A function and its inverse usually share the same physical key. One function is accessed directly, the other by using the SHIFT or 2ndF key.

The inverse of log functions are power functions.
Inv of ln is exponential (e^x)
Inv of log in base 10 is 10^(x)
A function and its inverse usually share the same physical key. One function is accessed directly, the other by using the SHIFT or 2ndF key.

Press [SHIFT][ division key]. It is marked with nCr.

This is a good scientific calculator that can do a lot, but things in the realm of a CAS (Computer Algebra System) are beyond its capabilities.

You should try to explore the calculator to find its capabilities. You can also read the manual. Everything is in it.
Press the Mode button, then the down arrow key. On the second page of the mode list you find 2:Verif

The inverse common log is the shifted function of the "log" key. The inverse natural log is the shifted function of the "ln" key.

The FX-991ES Plus barely handles simple operations on matrices of dimensions up to 3X3. It cannot find eigenvalues or eigenvectors.

Your calculator cannot perform inverse modulo calculations. To calculate yourself ( with pen and paper) you should consult a book on algebra.

Sorry, this calculator can handle matrices with dimensions ranging from 1X1 up to 3X3 . To deal with 4X4 matices you need a graphing calculator such as the CASIO FX-9570G Plus, the Ti83Plus.

The inverse of a natural logarithm (ln) is the exponetial (e^x). Both share the same physical key [LN] pour ln and [SHIFT] [LN] (e^x) for the exponential.
Syntax is : [ln] 3 [EXE] gives 1.098612289
exponential of 4: [SHIFT][LN] 4 [=] 54.59815003

The inverse of decimmal logarith is raise 10 to a power. Both share tha same physical key [LOG]
log of 6 : [log] 6 [=] result is 0.7781512504
antilog of 0.77: [SHIFT][LOG] 0.77 [=] result is 5.888436554

For more complicated arguments (expressions) it may be a good idea to use parentheses to enclose the arguments of functions.