Day 18 of 60 · Sat 17 Oct · Daily workbook · CAT in 43 days

Day 18: Progressions & Series + Chart DI Set + Politics & History RC

Day 18 of the free ThinkCAT 60-day CAT challenge: 5 progressions & series questions and 3 spaced-review QA questions, a full chart di DILR set (5 questions), a politics & history RC passage (5 questions), plus para jumbles and para summary. Every question is a real CAT PYQ.

QA · 8
DILR · 5
VARC · 7
45 minutes
Real CAT questions

Concept card

Progressions & Series

Core formulas

AP: aₙ = a + (n − 1)d; Sₙ = (n/2)(first + last)
Any series: aₙ = Sₙ − Sₙ₋₁
GP: aₙ = arⁿ⁻¹; S∞ = a/(1 − r) when |r| < 1
AM ≥ GM ≥ HM; Σn = n(n + 1)/2, Σn² = n(n + 1)(2n + 1)/6

60-second shortcut · Any term from the sum

Sₙ = 3n² + n ⇒ aₙ = Sₙ − Sₙ₋₁ = 6n − 2: the AP 4, 10, 16, … with d = 6.

Trap box

Looks like: S∞ = a/(1 − r) for any GPActually: Only when |r| < 1
Looks like: nth term of 3, 7, 11, … is 3 + 4nActually: a + (n − 1)d = 4n − 1

From the free Algebra Formula Sheet (PDF).

What's in Day 18

QA: Progressions & Series (5) + 3 spaced-review questions
DILR: Chart DI set, 5 questions
RC: Politics & History passage
VA: Para Jumbles, Para Summary

Year, slot and question number for each question appear with the answer key after you submit.

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