Which represents the measures of all angles coterminal with a 400° angle?

Coterminal angles differ by any integer multiple of 360°. Since 400° = 40° + 360°, all such angles can be written as 40 + 360n, where n is any integer. Integers include negative values, so this expression accounts for coterminal angles in both directions; the choice using whole numbers would omit some of them.

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