45

45 is the natural number that follows 44 and precedes 46. It is a composite number with prime factorization 3^2 × 5. Its divisors are 1, 3, 5, 9, 15, and 45.

In mathematics, 45 is the 9th triangular number since 1 + 2 + ... + 9 = 45. It is also a Harshad number, being divisible by the sum of its digits (4 + 5 = 9; 45 ÷ 9 = 5). In various numeral systems, 45 is written as XLV in Roman numerals, 101101 in binary, and 2D in hexadecimal.

In science, the atomic number 45 corresponds to rhodium (Rh), a rare, silvery-white transition metal. Rhodium is valued for its hardness, corrosion resistance, and high-reflectivity coatings, and it is widely used as a catalyst and in jewelry plating.

In culture and everyday life, 45 is associated with a 7-inch vinyl single played at 45 revolutions per minute and with the .45 caliber pistol and related ammunition (the term caliber .45 commonly refers to ammunition with roughly 0.45 inch diameter). A 45-degree angle is a common reference in geometry and drafting.

Regionally and historically, +45 is the international calling code for Denmark, and the 45th parallel north is a circle of latitude at 45 degrees north that passes through several continents. Notable uses include the designation of the 45th President of the United States, Donald J. Trump, and the year 1945, marking the end of World War II.

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