Common Mistakes

---
name: common-mistakes
description: Top mistakes students make in EACH chapter. Not categories (that's mistake-dna) but SPECIFIC errors like "forget state symbols in displacement reactions" or "use wrong sign in lens formula."
---

# Common Mistakes Per Chapter — Where Marks Actually Die

## Mathematics

### Quadratic Equations
```
❌ Forgetting ± in quadratic formula → lose 1M (only one root found)
❌ Wrong discriminant sign: writing b²+4ac instead of b²-4ac
❌ Not checking if D≥0 before solving → writing imaginary roots
❌ Word problems: setting up wrong equation from English statement
```

### Trigonometry
```
❌ sin 30° = √3/2 (WRONG — that's sin 60°). sin 30° = 1/2
❌ Identity proofs: working on BOTH sides simultaneously (prove from ONE side)
❌ Forgetting to convert units in height/distance problems (m vs cm)
❌ Using tan when question needs sin or cos (misreading the triangle)
```

### Statistics
```
❌ Using wrong formula: mode formula applied to mean question
❌ Cumulative frequency: adding wrong column in median calculation
❌ cf = cumulative frequency of class BEFORE median class, not the median class itself
❌ Forgetting to halve n for median (using n instead of n/2)
```

### Mensuration
```
❌ Using diameter instead of radius (forgetting to divide by 2)
❌ Frustum: confusing which radius is r₁ and which is r₂
❌ Leaving answer without units or wrong units (cm² vs cm³)
❌ π: using 3.14 when question says "use π = 22/7" (or vice versa)
```

## Science

### Chemical Reactions
```
❌ Unbalanced equations (examiner gives 0 for unbalanced)
❌ Missing state symbols: (s), (l), (g), (aq) — 0.5-1M deducted
❌ Writing Na₂ instead of 2Na (coefficient vs subscript confusion)
❌ Wrong reaction type label: calling decomposition "displacement"
```

### Electricity
```
❌ Sign confusion: V=IR gives R=V/I, NOT R=I/V
❌ Series vs parallel: using wrong formula for wrong configuration
❌ Units: writing Ω when answer should be in A (amps)
❌ Drawing ammeter in parallel / voltmeter in series (swapped positions)
```

### Light & Optics
```
❌ Sign convention errors: u is ALWAYS negative for real objects
❌ Magnification formula: using m=v/u instead of m=-v/u (missing negative)
❌ Convex lens image: wrong side of lens for virtual image
❌ Power formula: using f in cm instead of metres (P = 1/f in m)
```

### Life Processes
```
❌ Arteries carry "oxygenated blood" → WRONG for pulmonary artery
❌ Writing "veins carry deoxygenated blood" without exception note
❌ Confusing villi (intestine) with alveoli (lungs)
❌ Nephron diagram: missing glomerulus label
```

### Heredity
```
❌ Phenotypic ratio vs genotypic ratio (3:1 vs 1:2:1)
❌ Tt × Tt cross: forgetting to show Punnett square (lose 1M)
❌ Sex determination: XX=female, XY=male (not the other way)
❌ Writing "genes" when meaning "alleles" (they're different)
```

## Social Science

### History
```
❌ Wrong dates: Dandi March = 1930 (not 1920 or 1928)
❌ Confusing Non-Cooperation (1920) with Civil Disobedience (1930)
❌ Writing "Mahatma Gandhi" for European history questions
❌ Source-based: answering from memory instead of from the source
```

### Geography
```
❌ Map work: marking location in wrong state
❌ Confusing kharif (monsoon) with rabi (winter) crops
❌ Writing "black soil" for alluvial soil regions
❌ Missing specific data: "India has X% of world's land area" (exact number needed)
```

### Economics
```
❌ HDI: confusing high HDI with high GDP (they're different)
❌ Per capita income: forgetting to divide GDP by population
❌ Formal vs informal sector: labeling moneylender as "formal"
❌ WTO: writing "World Trade Organisation" but describing IMF functions
```

## English
```
❌ Formal letter: missing sender's address or date (1M lost on format alone)
❌ Tense conversion: "said" → reported speech but forgetting time shift
❌ Unseen passage: answering with outside knowledge (answer from passage ONLY)
❌ Analytical paragraph: writing personal opinions instead of data analysis
```