Mathematics Stack Exchange is a question and answer site for people studying math at any level and professionals in related fields. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

Find the last number in the given sequence $$\begin{pmatrix} 4& 9& 20\\ 8& 5& 14\\ 10& 3& ?\end{pmatrix}$$

(It's $3\times3$ matrix)

share|cite|improve this question
$\pi$.${}{}{}{}$ – Asaf Karagila Nov 19 '12 at 18:37
don't understand. – Patrick Li Nov 19 '12 at 18:58
@PatrickLi It's a joke. Like in: What are the two next terms in the sequence 1, 2, 4, 8, 16? Answer: 31 and 57. Reason: $a_n$ is the maximal number of pieces you can cut a cake into with $n-1$ straight slices.....when the cake is convex, 4-dimensional, and with nonempty interior. – Per Manne Nov 19 '12 at 19:09
Unless you give us a condition for determining what a "correct" answer is, there is no correct answer. – Thomas Andrews Nov 19 '12 at 19:21
@Asaf Sorry but you are wrong. Deadly wrong. How many times must I repeat that the answer is 42? Always. – Did Nov 19 '12 at 21:53
up vote 6 down vote accepted

The answer is $11$ since then $3 r_2 -r_1=2 r_3$ where $r_1, r_2, r_3$ are the three rows.

share|cite|improve this answer
I think this is the intended answer. – Ross Millikan Nov 19 '12 at 22:06
$11$ is the likely intended answer: another route using the columns would be $c_1+4c_2 = 2c_3$ as in my matrix multiplication. – Henry Nov 19 '12 at 22:51

One possibility is

$$\begin{pmatrix} 4& 9\\ 8& 5\\ 10& 3\end{pmatrix} \begin{pmatrix} 1& 0& \tfrac12\\ 0& 1& 2\end{pmatrix}$$

share|cite|improve this answer
Very clever answer! +1 – Joe Johnson 126 Nov 19 '12 at 19:37
How is that second matrix related to the problem? – hjg Nov 19 '12 at 22:41
It produces the first two terms in the third column of the problem and so is a possible solution to the question. There are more. – Henry Nov 19 '12 at 22:47

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.