Common Mistakes

---
target: 495+/500
author: Senior Sibling
name: common-mistakes
description: >-
  Top 12th Grade Mistakes per Unit. 
  Focus on Calculus precision, Organic mechanisms, Physics sign conventions, and Biotech terminology.
---

# Common Mistakes Per Chapter — The 12th Grade "Mark-Killers"

## 📐 Mathematics

### Integrals & Calculus
- ❌ **The Constant of Integration ($+C$):** Forgetting $+C$ in indefinite integrals. Examiners deduct 0.5m EVERY time.
- ❌ **Property Labeling:** Using the 'King Property' ($∫ f(a-x)$) without stating it.
- ❌ **Differential Eq:** Finding the wrong Integrating Factor ($e^{∫P dx}$) by misidentifying $P$.
- ❌ **Limits:** Evaluating $\int_a^b$ but substituting $a$ before $b$ (Upper - Lower!).

### Vectors & 3D Geometry
- ❌ **Shortest Distance:** Forgetting the magnitude in the denominator $|\vec{b1} \times \vec{b2}|$.
- ❌ **Dot vs Cross:** Using $\cos \theta$ for cross product or $\sin \theta$ for dot product.
- ❌ **Plane Equation:** Forgetting that $n$ is the **Normal** vector, not a vector on the plane.

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## 🔭 Physics

### Electrostatics & Current
- ❌ **Units:** Writing $10\mu C$ in a formula without converting to $10 \times 10^{-6} C$.
- ❌ **Gauss Law:** Forgetting the $2$ in the flux through a cylinder end-cap ($2EA$).
- ❌ **Kirchhoff's:** Wrong sign for $IR$ when moving against the current.

### Optics (The Biggest Mark Leak)
- ❌ **Sign Convention:** Forgetting that $u$ is ALWAYS negative.
- ❌ **Ray Diagrams:** Drawing rays without ARROWS. Arrows are mandatory for 3-5 marks.
- ❌ **Lens Maker:** Using $n2 - n1$ but forgetting to divide by $n1$ (i.e., $(n2/n1 - 1)$).
- ❌ **Power:** Using $f$ in $cm$ in the formula $P = 1/f$. $f$ MUST be in meters.

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## 🧪 Chemistry

### Organic Chemistry
- ❌ **Aldol vs Cannizzaro:** Attempting Aldol on an aldehyde with NO alpha-hydrogen (e.g., Benzaldehyde).
- ❌ **Mechanism Arrows:** Drawing arrows from positive to negative. Arrows ALWAYS show **electron** flow (negative $\to$ positive).
- ❌ **Distinction Tests:** Writing the observation without the reagent name.

### Physical & Inorganic
- ❌ **Nernst Equation:** Using $\ln$ instead of $\log$ (or forgetting the $2.303$ factor).
- ❌ **VBT:** Identifying an octahedral complex as $sp^3d^2$ (outer) when it should be $d^2sp^3$ (inner).
- ❌ **Kinetics:** Using the Zero-order half-life formula for a First-order question.

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## 🧬 Biology

### Genetics & Biotech
- ❌ **Terminology:** Confusing **Translation** (RNA to Protein) with **Transcription** (DNA to RNA).
- ❌ **Lac Operon:** Stating that Lactose is the "Repressor" (Lactose is the **Inducer**).
- ❌ **PCR:** Swapping the temperatures for Annealing and Extension.
- ❌ **Pedigree:** Confusing Autosomal Dominant with X-linked Recessive.

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## 💻 Computer Science
- ❌ **Pickle Module:** Using `f.write()` instead of `pickle.dump(data, f)` for binary files.
- ❌ **SQL:** Using `WHERE` after `GROUP BY`. You MUST use `HAVING` for groups.
- ❌ **Indentation:** Incorrect indentation in the `Push/Pop` stack logic.

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## 📖 English Core
- ❌ **Job Application:** Forgetting to attach the Bio-data/Resume after the cover letter.
- ❌ **Notice:** Writing the date in a confusing format (e.g., 10/12/26). Use "10th December 2026".
- ❌ **Literature:** Confusing the themes of "The Rattrap" (essential goodness) with "The Enemy" (humanity over war).