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MATH / The double factorial, denoted by n!!, returns the product of all of the odd integers that are less than or equal to n. For example, 7!!=7×5×3×1. What is the units digit of …

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problem

The double factorial, denoted by n!!n!!, returns the product of all of the odd integers that are less than or equal to nn. For example, 7!!=7×5×3×17!! = 7 \times 5 \times 3 \times 1. What is the units digit of 1!!+3!!+5!!+7!!+⋯+49!!1!! + 3!! + 5!! + 7!! + \cdots + 49!!?
Plain-text mathematical notation (without MathML)
The double factorial, denoted by n!!, returns the product of all of the odd integers that are less than or equal to n. For example, 7!!=7×5×3×1. What is the units digit of 1!!+3!!+5!!+7!!+⋯+49!!?
Original LaTeX notation
The double factorial, denoted by $n!!$, returns the product of all of the odd integers that are less than or equal to $n$. For example, $7!! = 7 \times 5 \times 3 \times 1$. What is the units digit of $1!! + 3!! + 5!! + 7!! + \cdots + 49!!$?

level

Level 5

type

Number Theory

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