Friday, October 22, 2010

TANCET Quant : Progressions

AP, GP (Arithmetic Progressions and Geometric Progressions) are two topics from which you could expect to get at least one question.

An interesting topic and the kind of questions that you can expect in the TANCET quant section from these two topics will be rather elementary.

The AP topic has two formulae to remember.

The first one is to find the nth or the last term 'l' of an AP : l = a + (n - 1)d, where 'a' is the first term, n is the number of terms and 'd' is the common difference.

The second formula is to find the sum of first 'n' terms of an AP. Sn = (n/2)(2a + (n - 1)d)

Here is a question that tests your understanding of these concepts.

If the first term of an AP is 12 and the 7th term is 36, what is the sum of these 7 terms?
1. 240
2. 168
3. 336
4. 84
5. 121

The first term a = 12
The 7th term l = 36.

We know that l = a + (n - 1)d
or 36 = 12 + (7 - 1)d
or 24 = 6d
or d = 4.

Sum of these first 7 terms = (n/2)(a + (n - 1)d), where n = 7, a = 12, d = 4

Substituting we get, (7/2)(2*12 + (7 - 1)4)
= (7/2)(24 + 24)
= 7 * 24
= 168

Choice (2) is the correct answer.