Click here to win prizes by solving the puzzle Advent calendar.
Click here to win prizes by solving the puzzle Advent calendar.



Triangle numbers

Let \(T_n\) be the \(n^\mathrm{th}\) triangle number. Find \(n\) such that: $$T_n+T_{n+1}+T_{n+2}+T_{n+3}=T_{n+4}+T_{n+5}$$

Show answer & extension

If you enjoyed this puzzle, check out Sunday Afternoon Maths VI,
puzzles about triangle numbers, or a random puzzle.


Show me a random puzzle
 Most recent collections 

Sunday Afternoon Maths LXVII

Coloured weights
Not Roman numerals

Advent calendar 2018

Sunday Afternoon Maths LXVI

Cryptic crossnumber #2

Sunday Afternoon Maths LXV

Cryptic crossnumber #1
Breaking Chocolate
Square and cube endings

List of all puzzles


irreducible numbers parabolas probabilty cryptic crossnumbers probability dates shape perimeter complex numbers cards sum to infinity graphs cryptic clues proportion circles christmas people maths division squares unit fractions balancing square numbers coordinates rugby means averages speed remainders spheres star numbers triangles money wordplay chocolate scales angles pascal's triangle 3d shapes addition percentages sport doubling planes 2d shapes ellipses floors dice ave triangle numbers logic sums books crosswords folding tube maps partitions fractions time geometry multiplication taxicab geometry crossnumbers cube numbers palindromes colouring lines surds mean games volume sequences prime numbers dodecagons odd numbers shapes numbers trigonometry differentiation bases perfect numbers quadratics factorials arrows integration indices chess rectangles multiples functions calculus symmetry integers chalkdust crossnumber routes factors advent regular shapes area hexagons number square roots coins menace digits algebra grids clocks polygons


Show me a random puzzle
▼ show ▼
© Matthew Scroggs 2012–2019