\"\"

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The list is \"\"

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Observe the pattern, the difference between consecutive numbers is 4.

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\"\"

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\"\"

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Let n be the next term.

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Write an equation for the situation.

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Previous term \"\" 4 = Next term.

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\"\"               (Substitute previous term = 21 and next term = n)

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\"\"                    (Subtract: \"\")

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The extended pattern is

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\"\"

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The next term in the given list is 17.         

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\"\"

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The next term in the given list is 17.