55 (number)
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55 is a natural number used in standard mathematics. It follows 54 and precedes 56. It is an odd number, meaning it cannot be divided by two without a remainder.
55 is a composite number, being the product of two prime numbers: 5 × 11. Its divisors are 1, 5, 11, and 55. In Roman numerals it is written as LV.
Mathematical properties
55 is a triangular number. It equals the sum of the integers from 1 to 10: 1+2+3+…+10 = 55. It is therefore the tenth triangular number.
It is also a square pyramidal number: the sum of the first five perfect squares, 1+4+9+16+25 = 55. Numbers that are both triangular and square pyramidal are very rare.
55 is a member of the Fibonacci sequence, in which each number is the sum of the two preceding numbers: 1, 1, 2, 3, 5, 8, 13, 21, 34, 55. It is the tenth term when starting from 1.
Common uses
55 commonly appears in speed limits on roadways (55 km/h or 55 mph in certain countries), in age ranges for working-age adults, and in television screen measurements (55 inches measured diagonally).