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Spot the mistake relay and maths taboo for CBSE Class 10

An end-of-term lesson that is fun and still useful: teams race to find the slip in worked solutions built on the most common board exam mistakes, then play maths taboo and pictionary with the year’s vocabulary.

Level
CBSE Class 10 (Class 9 can try most of it)
Time
39 minutes, plus a 10-minute extension
Topics
Quadratic equations and APs; Areas related to circles and statistics; Probability and trigonometry
Equipment
No calculator. Use π = 22/7 where a question says so.

Download student sheet (PDF)Answers (PDF)

Suggested timings

PartTimeWhat
Starter5 minQuick questions on the board
Main: task A12 minRelay, part 1
Main: task B12 minRelay, part 2
Main: task C10 minMaths taboo
Extension10 minFast finishers or homework

Starter (5 minutes)

Spot the slip: each line has one mistake.

  1. (x + 3)2 = x2 + 9
  2. When x = −3, 3x2 − 2x = 27 − 6 = 21
  3. In the AP with a = 5 and d = 3, a10 = 5 + 10 × 3 = 35

Main activity (34 minutes)

Task A: Relay, part 1 (12 min)

Each worked solution has exactly one mistake. Find it, then give the correct answer.

  1. Find the area of a circle of diameter 14 cm. (Use π = 22/7.) Working: π × 142 = 22/7 × 196 = 616 cm2.
  2. Class intervals 0–10, 10–20, 20–30, 30–40 have frequencies 12, 3, 7, 8. Find the median. Working: the class with the highest frequency is 0–10, so the median class is 0–10.
  3. Two coins are tossed. Find P(exactly one head). Working: the outcomes are 2 heads, 1 head, 0 heads, so P = 1/3.

Task B: Relay, part 2 (12 min)

Each worked solution has exactly one mistake. Find it, then give the correct answer.

  1. Solve x2 − 5x − 14 = 0. Working: (x − 7)(x + 2) = 0, so x = −7 or x = 2.
  2. Find the sum of the first 20 terms of 4, 7, 10, … Working: S = 20/2 × (2 × 4 + 20 × 3) = 10 × 68 = 680.
  3. Evaluate sin 60° cos 30° + cos 60° sin 30°. Working: (√3/2)(√3/2) + (1/2)(1/2) = 3/4 + 1/2 = 5/4.

Task C: Maths taboo (10 min)

Describe the word to your team without saying it or any of the banned words.

  1. Describe PERIMETER without saying: around, edge, outside, length.
  2. Describe PRIME without saying: factor, divide, one, itself.
  3. Describe MEDIAN without saying: middle, average, centre, order.
  4. Describe ZERO OF A POLYNOMIAL without saying: root, solve, nothing, equals.

Extension (10 minutes)

For fast finishers.

  1. Find the roots of x2 − 6x + 9 = 0. Working: D = 36 − 36 = 0, so there are no real roots.

For teachers

Teacher notes and full worked answers
  • Relay: teams of three. Give each team the first worked solution; when they bring you the mistake and the corrected answer, they get the next one. The first team through all six wins.
  • Every slip here is one of the common mistakes on our Class 10 Examiner Insights page, in a new example of our own. Show the page afterwards so students can see which mistakes cost the most marks.
  • Taboo cards: one student describes the word without saying it or any of the banned words; their team guesses. The sample clue on each card shows that it can be done.
  • Pictionary (no words or numbers allowed on the board, 60 seconds each): parallel, perpendicular, tangent, frustum, angle of elevation, sector, histogram, cylinder.

Starter

  1. x2 + 6x + 9
    • (a + b)2 = a2 + 2ab + b2
  2. 33
    • −2 × (−3) = +6, so 27 + 6 = 33
  3. 32
    • an = a + (n − 1)d = 5 + 9 × 3

Task A: Relay, part 1

  1. 154 cm2
    • Mistake: the diameter was squared instead of the radius.
    • r = 7: 22/7 × 49 = 154
  2. 20
    • Mistake: that is the modal class. For the median use n/2 = 15 and the cumulative frequencies 12, 15, 22, 30: the median class is 10–20.
    • Median = 10 + (15 − 12)/3 × 10 = 20
  3. 1/2
    • Mistake: those three outcomes are not equally likely.
    • HH, HT, TH, TT: two of four have exactly one head, so P = 1/2.

Task B: Relay, part 2

  1. x = 7 or x = −2
    • Mistake: a sign slip at the last step.
    • x − 7 = 0 gives x = 7; x + 2 = 0 gives x = −2.
  2. 650
    • Mistake: the formula uses (n − 1)d, not nd.
    • S = 10 × (8 + 19 × 3) = 10 × 65 = 650
  3. 1
    • Mistake: (1/2)(1/2) = 1/4, not 1/2.
    • 3/4 + 1/4 = 1

Task C: Maths taboo

  1. Sample clue: ‘Add up all the sides of a shape.’
    • The word is perimeter. Any clue that avoids the banned words is fine.
  2. Sample clue: ‘Numbers like 2, 3, 5, 7 and 11.’
    • The word is prime. Any clue that avoids the banned words is fine.
  3. Sample clue: ‘Put the data in a row from smallest to largest and pick the value halfway along.’
    • The word is median. Any clue that avoids the banned words is fine.
  4. Sample clue: ‘The x-value where the graph of p(x) cuts the x-axis.’
    • The word is zero of a polynomial. Any clue that avoids the banned words is fine.

Extension

  1. x = 3 (two equal roots)
    • Mistake: D = 0 means two equal real roots; D < 0 means no real roots.
    • (x − 3)2 = 0, so x = 3.

The same answers are in the answers PDF. Every answer was recomputed by computer before publishing.

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