Concours Centrale-Supélec common mistakes
Concours Centrale-Supélec · Understand the traps and how to avoid them.
In the written mathematics paper, the candidate launches into calculations without first identifying the structure of the problem (e.g., a linear recurrence, an integral depending on a parameter, or a sequence defined implicitly), and then gets lost in computation and runs out of time.
Under time pressure, candidates tend to react to the first formula they see and start manipulating symbols, skipping the preliminary analysis of hypotheses and the goal of the question. The Centrale mathematics papers often chain questions where each result is needed later, so a blind start leads to dead ends.
What to do instead
Spend the first 5–10 minutes reading the whole problem and writing a one-line summary of each question and its dependencies. Before computing, state the type of object, the hypotheses, and the result to be proved. If a question resists for more than 10–15 minutes, note the admitted result and move on, returning later only if time permits.
In physics, the candidate writes numerical applications with inconsistent units or significant figures, or forgets to check the order of magnitude and the homogeneity of the formula.
Candidates focus on getting the algebra right and treat units and orders of magnitude as cosmetic. In Centrale physics papers, numerical results are often worth marks, and an absurd value (e.g., a speed greater than c, a temperature of 10^7 K for a lab experiment) signals an error that could have been caught.
What to do instead
Systematically perform a dimensional analysis before substituting numbers, keep units throughout the calculation, and write the final result with a sensible number of significant figures. Add a one-line sanity check (order of magnitude, limiting case, comparison with a known value) for every numerical answer.
In sciences industrielles de l'ingénieur, the candidate produces a kinematic or dynamic analysis without clearly defining the reference frames, the parametrization, and the closure equations, leading to inconsistent results and an inability to answer the final performance question.
The complexity of mechanisms (e.g., a robot arm, a gear train) pushes candidates to jump to formulas for speed ratios or torques without setting up the geometric model. The Centrale SII papers require a structured approach: graphe des liaisons, schéma cinématique, fermeture géométrique, then derivation.
What to do instead
Always start with the graphe des liaisons and the schéma cinématique minimal, then write the vector loop (fermeture géométrique) explicitly before differentiating. Name every frame and parameter. Check the number of degrees of freedom against the expected mobility before proceeding to dynamics.
In chemistry, the candidate writes balanced equations and equilibrium expressions without verifying the conservation of elements and charge, or applies a formula (e.g., pH of a weak acid) outside its validity domain (e.g., when the approximation is not justified).
Candidates memorize formulas and apply them mechanically, especially in acid-base and redox problems. Centrale chemistry questions often test the validity conditions (e.g., degree of dissociation, ionic strength) and penalize unjustified approximations.
What to do instead
For every equation, explicitly check atom and charge balance. Before using an approximate formula, state the condition (e.g., α < 0.05, c > 10^-6 M) and verify it numerically. If the condition fails, solve the full equation or use a systematic method (tableau d'avancement, équations de conservation).
In the français dissertation, the candidate produces a descriptive list of examples or a catalogue of authors instead of constructing a dialectical argument that answers the question, and neglects the transition between parts.
Candidates often prepare stock examples and try to place them regardless of the topic. The Centrale dissertation requires a clear problématique, a thesis-antithesis-synthesis structure, and a conclusion that answers the question. Without this, even a well-written copy gets a mediocre mark.
What to do instead
Spend 20–30 minutes on the plan: formulate the problématique as a tension, define two or three parts each with a clear argumentative function, and select two or three precise examples per part. Write explicit transitions that explain the logical progression. In the introduction, announce the plan; in the conclusion, answer the question explicitly.
In the résumé (summary) exercise in French, the candidate copies sentences from the text, exceeds the word limit, or includes personal commentary, instead of reformulating the ideas in a neutral, condensed form with the required proportion.
Under time pressure, candidates paraphrase too closely or paste fragments, and lose marks on reformulation and on the strict word count (often 1/4 of the original). The Centrale summary also requires respecting the order of ideas and the enunciation of the author.
What to do instead
Read the text twice, identify the thesis and the main argumentative steps, and write the summary from your notes without looking at the original. Count the words precisely (including articles) and stay within ±10%. Use reported speech (l'auteur affirme que…) and avoid any personal judgment.
In the written expression in a modern language, the candidate translates literally from French, uses inappropriate register, and fails to structure the essay (introduction, body, conclusion) or the summary (respecting the word limit).
Candidates rely on French syntax and vocabulary calques, and often ignore the conventions of the exercise (e.g., a 200-word essay, a summary in the target language). The Centrale language papers test both linguistic accuracy and argumentative structure.
What to do instead
Prepare a repertoire of connectors and essay phrases in the target language. Before writing, outline the structure in note form. After writing, spend 5 minutes checking verb tenses, agreement, and false friends. For summaries, count words and verify that you have not copied more than three consecutive words from the original.
In the oral mathematics examination, the candidate starts writing immediately without taking time to understand the question, does not interact with the examiner, and becomes silent when stuck instead of explaining the blockage.
Oral stress leads to a rush to produce something on the board, and candidates fear that silence is penalized. But the Centrale oral is a dialogue: the examiner expects the candidate to propose an approach, test it, and react to hints. A silent candidate cannot be helped.
What to do instead
Take 1–2 minutes to read the question and state a plan aloud (e.g., 'I will first try to prove…'). Write clearly and explain each step. If stuck, say what you are trying, what fails, and ask a precise question. Treat hints as information to integrate, not as a rescue.
In the oral physics or engineering examination, the candidate performs calculations without commenting on the physical meaning, does not justify approximations, and fails to connect the model to the experimental or industrial context.
Candidates reproduce the written-paper reflex of calculating for the sake of calculating. The oral examiner tests whether the candidate can interpret a model, discuss its validity, and propose an experimental check. A correct calculation without physical commentary gets limited credit.
What to do instead
For each result, state what it represents, its order of magnitude, and its limiting cases. Justify every approximation (e.g., small oscillations, ideal gas) and say when it breaks down. End each question by proposing a way to verify the result experimentally or numerically.
In the oral language examination, the candidate delivers a memorized monologue on a prepared topic, ignores the document or the question asked, and is unable to react to the examiner's follow-up questions.
Candidates prepare stock presentations and hope the topic matches. The Centrale oral in languages often includes a document to comment on and a discussion; a rigid monologue signals a lack of comprehension and interaction skills.
What to do instead
Practice commenting on unseen documents: identify the source, the type, the main idea, and two or three striking details, then give a personal reaction. Prepare flexible vocabulary for reacting (e.g., 'I see your point, however…'). In the exam, listen carefully to the question and answer it directly before developing.