Where do i find the reciprocal key on the casio fx-115ES

Once you create a non-singular square matrix, have its name displayed on the command line then press the reciprocal key.

The Casio FX-570MS does not handle matrices. So you cannot create matrices, let alone invert them. Sorry.
The Casio FX115 ES handles matrices for up to 3X3 dimension. Create the matrix.Have its name displayed on the command line and press the [x^-1] (reciprocal ) key.

It's the shifted function of the x^-1 key, located just below and right of the big round cursor pad, above the log key.

For the reciprocal of a number, enter the number then press the key marked with an x^-1. You might have to use the SHIFT key first. For inverse of trigonometric functions use the SHIFT key followed by the function name. The inverse of LN is e^x. The inverse of log is 10^x. To calculate the inverse of a matrix, enter the name of the (already created) square matrix then press x^-1.

http://support.casio.com/manualfile.php?rgn=1&cid=004001004

Not all matrices are invertible. The matrice must be square.The matrix must be non-singular: Its determinant must be different from 0. Your error comes probably from the key you used to calculate the inverse. Use the reciprocal key, the one marked with x^-1. If the marking is not ON the key, use the SHIFT button prior to pressing the relevant key.

It's the key marked 1^-1, just to the right and below the big circular cursor pad, above the log key.

Hello,
To convert fractions to decimals or decimals to fractions, you use the [S<->D] key. Each time you press the key the result is displayed in the other format.
Alternatively, If you multiply or divide two integer numbers put a decimal mark at the end of one of the numbers (any one). The result will be displayed in decimal format.

Hope it helps.

Use the key that is 3 keys below the ALPHA button. You must press that key, then place the number or equation you are trying to get the square root of.

x + 1) = 7