Here are some usages of the Casio Calculator 991 ES Plus ¹ ² ³:

- Basic arithmetic operations: Perform addition, subtraction, multiplication, and division.

- Fraction calculations: Perform calculations with fractions.

- Statistical operations: Perform statistical calculations like standard deviation and regression.

- Matrix calculations: Perform matrix calculations and solve simultaneous equations.

- Trigonometric calculations: Calculate sin, cos, tan and their inverse.

- Logarithmic calculations: Calculate logarithmic functions and natural logarithm base e.

- Hyperbolic calculations: Calculate hyperbolic, inverse hyperbolic functions and hyperbolic angles.

- Angle conversion: Convert between different angle units like degrees, minutes, seconds and radians.

- Exponential calculations: Calculate powers, roots, exponential functions and exponentials.

- Complex number calculations: Calculate complex numbers.

- Base-n calculations: Perform calculations using decimal, hexadecimal, binary and/or octal values.

- Integration calculations: Calculate integrals.

- Differential calculations: Calculate derivatives.

- Vector calculations: Perform vector calculations.

- Inequality calculations: Solve inequalities.