The Casio Scientific Calculator FX-991ES Plus is one of the most popular and feature-rich non-programmable scientific calculators available, widely used by high school and university students in fields like engineering, mathematics, and physics.1It is often recognized for its Natural Textbook Display and extensive set of functions.2Key Features and SpecificationsFeatureDetailDisplayNatural Textbook Display (Natural V.P.A.M.): Shows mathematical expressions like roots, fractions, and exponents exactly as they appear in a textbook, which aids comprehension.Functions417 Functions (or sometimes 401, depending on the edition/region).Power SourceTwo-Way Power (Solar & Battery): Solar powered when light is sufficient, and battery powered (usually 1 x LR44) when light is insufficient.Core CalculationsComprehensive functions including trigonometric, logarithmic, exponential, reciprocal, and factorial calculations.Advanced MathNumerical Integration ($\int$) and Differential Calculus ($\frac{d}{dx}$), Base-n (binary, octal, hexadecimal), Logical Operators.Algebra & EquationsEquation Calculations (Simultaneous linear equations up to 3 unknowns, quadratic, and cubic equations). CALC and SOLVE functions for solving equations.Vectors/MatricesMatrix Calculations (up to 3x3) and Vector Calculations (up to 3 dimensions).StatisticsList-based STAT data editor, Standard Deviation, Regression Analysis, Permutations ($nPr$) and Combinations ($nCr$).Engineering/Science40 Scientific Constants and Metric Conversion Function (20 conversion pairs).Memory9 Variable Memories, Answer Memory, and Independent Memory.DesignDot Matrix Display (10-digit mantissa + 2-digit exponent), comes with a slide-on hard case.