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U{n} = (-52n⁵ + 795n⁴ -4510n³ + 12045n² - 14518n + 6720)/120

Which gives U{1-5} = {4, 7, 13, 25, 49} and U6 = 42.

There are an infinite number of polynomials which will give the given sequence when the values 1-5 are input.

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What your teacher is most likely expecting is:

U{n} = 2.5 + 1.5 × (2ⁿ - 1)

or U{n} = (5 + 3 × (2ⁿ - 1)) / 2

Discovered via:

U2 = U1 + 3

U3 = U2 + 3 × 2

U4 = U3 + 3 × 2²

U{n} = U{n-1} + 3 × 2ⁿ⁻²

→ U1 = U0 + 3 × 2⁻¹

→ U0 = U1 - 3 × 2⁻¹

= 4 - 3/2

= 2.5

→ U1 = U0 + 3 × 2⁻¹ = 2.5 + 3 × 2⁻¹

U2 = U1 + 3 × 2⁰ = 2.5 + 3 × 2⁻¹ + 3 × 2⁰

U3 = U2 + 3 × 2¹ = 2.5 + 3 × 2⁻¹ + 3 × 2⁰ + 3 × 2¹

→ for n = 1, 2, 3, ...

U{n} = 2.5 + 3 × 2⁻¹ + 3 × 2⁰ + 3 × 2¹ + ... + 3 × 2ⁿ⁻²

= 2.5 + 3 × 2⁻¹ × (1 + 2¹ + 2² + ... + 2ⁿ⁻¹)

= 2.5 + 1.5 × (2ⁿ - 1)/(2 - 1)

= 2.5 + 1.5 × (2ⁿ - 1)

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