Calculator guides

Casio fx-991 ClassWiz

GCSE & A-level

Advanced scientific calculator · organised by the exam question that needs it.

The advanced scientific calculator — fractions, standard form, statistics, the equation solver and tables of values, organised by the exam question that needs them.

Steps are for the ClassWiz fx-991 (EX and CW). On the fx-991EX the settings menu is SHIFT then SETUP; on the newer fx-991CW use the dedicated SETTINGS key (the menu names are the same). The equation solver and numerical calculus are ClassWiz features the basic GCSE fx-83GT/85GT models don't have — but those do have fractions, statistics, standard form and a table of values, with the same steps.

Number

Standard form (powers of ten)

When to use: Enter or calculate with numbers in standard form, e.g. a physics or number question written as a × 10ⁿ.

  1. 1Type the number part, e.g. 3.2
  2. 2Press the ×10ˣ key — This is the dedicated standard-form key — do NOT type '× 1 0 ^'.
  3. 3Type the power, e.g. 5, then = — 3.2 ×10ˣ 5 means 3.2 × 10⁵.
  4. 4To show answers in standard form: SHIFT SETUP → Number Format → Sci — On the fx-991CW use the SETTINGS key instead of SHIFT SETUP.

Work out (3 × 10⁵) × (2 × 10³).

3 ×10ˣ 5×2 ×10ˣ 3=

Answer: 6 × 10⁸

  • Using the ×10ˣ key keeps the whole '× 10 to the power' as one number, so the order of operations is right; typing 10^ separately is where marks get lost.
  • If the display shows a long decimal, switch Number Format to Sci to read it back as standard form.

Fractions and exact ↔ decimal

When to use: Keep a value as an exact fraction or surd, or convert between a fraction and its decimal.

  1. 1Use the fraction template key to enter a fraction (numerator, ▼, denominator)
  2. 2Press = to get the answer in its exact form
  3. 3Press the S⇔D key to toggle between the exact fraction/surd and the decimal

Write 0.375 as a fraction.

0.375=S⇔D

Answer: 3/8

  • S⇔D converts terminating decimals to fractions and surds to decimals — handy to check an exact answer.
  • If a 'give an exact answer' question shows a decimal, press S⇔D to get the fraction or surd the mark scheme wants.

Equations & algebra

Solve quadratics, cubics and simultaneous equations

When to use: Solve a quadratic/cubic or a set of simultaneous linear equations and you only need the answer (or a check).

  1. 1MENU → Equation (Equation/Func)
  2. 2Choose Simultaneous (2–4 unknowns) or Polynomial (degree 2–4)
  3. 3Type each coefficient, pressing = after each
  4. 4Press = to read out each solution

Solve x² − 5x + 6 = 0.

MENU → EquationPolynomial, degree 21 = , −5 = , 6 ==

Answer: x = 3 and x = 2

  • This is a ClassWiz feature — the basic GCSE fx-83GT/85GT cannot do it, so don't rely on it in a shared-calculator setting.
  • For 'solve algebraically' or 'show that', the calculator answer scores nothing without the factorising/formula/elimination working.

Table of values (for graphs and roots)

When to use: Fill in a table of values to draw a graph, or locate a root by looking for a change of sign.

  1. 1MENU → Table
  2. 2Type the function as f(X), then =
  3. 3Enter Start, End and Step when prompted
  4. 4Scroll the table to read each value — A sign change between two rows means a root lies between them.

Show that x² − 3 = 0 has a root between x = 1 and x = 2.

MENU → Tablef(X) = X² − 3, =Start 1, End 2, Step 1read f(1) and f(2)

Answer: f(1) = −2 and f(2) = 1: the sign changes, so there is a root between 1 and 2

  • The change-of-sign argument is the written method — quote both values and say the sign changes, don't just point at the calculator.
  • Use a small Step (e.g. 0.1) to trap a root more tightly for a decimal-search question.

Statistics

Mean and standard deviation

When to use: A list of data or a frequency table, and the question asks for the mean or standard deviation.

  1. 1MENU → Statistics → 1-Variable
  2. 2For a frequency table, first turn Frequency On — SHIFT SETUP → Statistics → Frequency: On (fx-991CW: use the SETTINGS key).
  3. 3Type the values (and their frequencies) into the table
  4. 4Press AC, then OPTN → 1-Variable Calc — Reads x̄ (mean), σx (population sd), sx (sample sd), Σx, n.

Find the mean and population standard deviation of 2, 4, 4, 4, 5, 5, 7, 9.

MENU → Statistics → 1-VariableEnter the eight valuesAC, then OPTN → 1-Variable Calc

Answer: mean x̄ = 5, population sd σx = 2 (sample sd sx ≈ 2.14)

  • σx is the population standard deviation; sx is the sample one — check which the question wants.
  • Turn Frequency On before entering a frequency table, or every value is counted once.

Regression line and correlation

When to use: Bivariate data: find the equation of the line of best fit y = a + bx, or the correlation coefficient r.

  1. 1MENU → Statistics → y = a + bx
  2. 2Type the x-values and y-values into the two columns
  3. 3Press AC, then OPTN → Regression Calc — Gives a, b and r.
  • r close to ±1 is strong linear correlation; close to 0 is weak.
  • Use the y = a + bx line to predict within the data range only — extrapolation is a written-answer trap.

Trigonometry

Degrees vs radians (and inverse trig)

When to use: Any trig calculation — and the single most common calculator error in the exam.

  1. 1SHIFT SETUP → Angle Unit → Degree or Radian — On the fx-991CW use the SETTINGS key instead of SHIFT SETUP.
  2. 2Check the small D or R indicator at the top of the screen
  3. 3For an angle from a ratio, use SHIFT then sin / cos / tan (sin⁻¹ etc.)
  • GCSE and A-level mechanics/geometry are in degrees; A-level trig/calculus questions are usually in radians — set the mode to match before you start.
  • If an answer is wildly wrong (e.g. an angle of 0.5 where you expected 30), you are almost certainly in the wrong angle mode.

Calculus (A-level)

Definite integral and gradient at a point

When to use: Check a definite integral, or the value of dy/dx at a particular x — the ClassWiz does both numerically.

  1. 1For an integral: press the ∫ key
  2. 2Enter the lower limit, upper limit and the expression, then = — e.g. ∫ from 0 to 2 of X² dX.
  3. 3For a gradient: press the d/dx key, enter the expression and the x-value, then =

Evaluate the integral of x² from 0 to 2.

∫ keylower 0, upper 2, expression X²=

Answer: 8/3 ≈ 2.667

  • These give a number, not the algebra — an A-level 'integrate' or 'differentiate' question still needs the worked method for the marks.
  • Great for checking your by-hand answer before you commit to it.

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