Problems

Age
Difficulty
Found: 20

Using mathematical induction prove that \[1 + 2 + 3 + \dots + n = \frac{n(n+1)}{2}.\]

There are \(n\) lines on a plane, and all the lines intersect at exactly one point. Prove that the lines divide the plane into \(2n\) parts.

There are \(n\) lines on a plane, no two lines are parallel, and no three lines cross at one point. Show that those lines dived the plane into \(\frac{n(n+1)}{2}+1\) regions.

In a sequence 2, 6, 12, 20, 30, ... find the number

(a) in the 6th place

(b) in the 2016th place.

Using mathematical induction prove that \[1 +3 +5 +\dots + (2n-1) = n^2.\]

Circles and lines are drawn on the plane. They divide the plane into non-intersecting regions, see the picture below.

Show that it is possible to colour the regions with two colours in such a way that no two regions sharing some length of border are the same colour.

Numbers \(1,2,\dots,n\) are written on a whiteboard. In one go Louise is allowed to wipe out any two numbers \(a\) and \(b\), and write their sum \(a+b\) instead. Louise enjoys erasing the numbers, and continues the procedure until only one number is left on the whiteboard.

What number is it? What if instead of \(a+b\) she writes \(a+b-1\)?

Prove that

(a) \[1^2 + 2^2 + 3^2 + \dots + n^2 = \frac{1}{6} n (n+1)(2n+1)\]

(b) \[1^2 + 3^2 + 5^2 + \dots + (2n-1)^2 = \frac{1}{3} n (2n-1)(2n+1).\]

Using mathematical induction prove that \(2^n>n\) for all natural numbers.

Using mathematical induction show that \(2^n>n\) for all natural numbers \(n\).