Some OCR questions begin "In this question you must show detailed reasoning." It is easy to read that as "show your working", shrug, and carry on. It means something more specific, and getting it wrong costs marks on a question you can do.

It appears on several questions in every paper, on both OCR A and OCR MEI, so you will meet it whether you like it or not.

In this guide

What the instruction means

OCR explains it to students like this:

It's always good practice to show workings, but the statement "In this question you must show detailed reasoning" emphasises there are marks allocated for workings. You can still use your calculator to check the answer though.

Two useful things come out of that.

You are not banned from using your calculator. Check your answer with it. Evaluate a function with it. What you cannot do is let it replace the working that earns the marks.

The marks are already there. This is the part students miss. Detailed reasoning does not add a bonus mark for explaining. It means the marks in that question will not be given unless the marker can see how you got there.

Put simply: on a normal question a correct answer usually implies the method. On a detailed reasoning question it does not. The right answer with nothing underneath it can score nothing.

The words to look for: "must be seen"

Read a few OCR mark schemes for these questions and one phrase comes up again and again: "must be seen". Mark schemes are published free on OCR's website, and this is the single most useful thing to notice in them.

Some real examples from OCR mark schemes:

  • "DR so method must be seen"
  • "Explicit use of limits must be seen in a correct integral"
  • "Some evidence of finding critical values must be seen"
  • "Sufficient working must be seen. Discriminant must have been set \( > 0 \) before dividing by 8"

That last one is worth reading twice. It is not enough to reach the right inequality. The examiner needed to see the discriminant set greater than zero before the tidying up, because that is the step being tested.

Say why you threw a solution away

This one is particular to OCR and it catches people out. Marks regularly depend on giving a reason for rejecting a solution, and OCR says so in its examiner guidance.

If you solve an equation and get two answers, one of which does not make sense in the question, do not simply write down the one you want. Say why the other one goes.

  • "\( t = -3 \) is rejected because time cannot be negative."
  • "\( x = -2 \) is outside the given domain \( 0 \leqslant x \leqslant 5 \)."
  • "This root is rejected because \( \ln x \) is undefined for \( x \leqslant 0 \)."

One short clause each time. OCR's own examiner guidance asks candidates to justify "giving a reason when discarding solutions to an equation that are not valid".

A worked example

Suppose a detailed reasoning question asks you to solve

\[ 3^{2x+1} = 4^{100} \]

giving your answer to three significant figures.

A calculator will hand you \( x = 62.6 \) in one step. On a detailed reasoning question that scores nothing, because none of the method is on the page. What is wanted is the chain:

\[ \log 3^{2x+1} = \log 4^{100} \]

\[ (2x+1)\log 3 = 100 \log 4 \]

\[ 2x + 1 = 126.18\ldots \]

\[ x = 62.6 \text{ (3 s.f.)} \]

OCR's guidance says using a solve function to skip any one of those lines "would not result in a complete analytical method". The intermediate value matters as much as the logarithm step.

The other OCR words that change the answer

OCR is unusual in publishing what its command words mean. Three are worth knowing because they change how much you need to write.

Determine, not find

These look like synonyms and are not.

  • Find, solve, calculate — you get full marks for the correct answer with no justification at all. OCR says so outright, and says the answer may come from a calculator or a graph.
  • Determine — you must justify the result. As OCR puts it, "you can't just state the answer, even if you can generate it from the calculator".

OCR uses determine in about a third of its questions, far more than any other board, so this is not a rare case.

Verify, not show that

  • Verify — substitute the given value in and show it works. You are confirming a claim, so this is quick.
  • Show that — you have been given the answer, so you must produce every step that reaches it.

Doing the show that amount of work on a verify question wastes time you need later in the paper.

Exact

Leave surds, fractions, \( \pi \), \( \mathrm{e} \) and logarithms alone. If a question asks for the exact solution of \( \ln x = 2 \), the answer is \( \mathrm{e}^2 \), not \( 7.389 \).

The mistake OCR names as the most common on "show that" questions is doing too many steps in one line. Their advice: do each key process on a new line. It is easier for you to check and easier for the examiner to award.

A checklist for the exam

When a question says you must show detailed reasoning:

  • Write one step per line. No combining, no jumping.
  • Show the step the question is testing, not just the line after it. If it is about a discriminant, the discriminant must appear.
  • Give a reason for any solution you reject. One clause is enough.
  • Use the calculator to check, not to answer. Checking is encouraged.
  • Read the first word of every other question too. "Determine" wants justification and "find" does not.

Go to the source. OCR publishes a free guide called "Exam hints for students" which explains every command word and this instruction, in plain language. Their specification has a command words section too. Both are on the OCR website and both are worth twenty minutes before your next mock.