Hint #1
First, notice the the expansion for one the sample inputs provided
Input:
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3 3
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1 x 3^1 + 2 x 3^2 + 3 x 3^3 = 3 + 18 + 81 = 102
Hint #2
More generally, using lower-case "a" instead of upper-case "A" (which is plain weird to use) this can be written out as
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a + 2 x a^2 + 3 x a^3 + ... + n x a^n
Hint #3
Let
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y = a + 2 x a^2 + 3 x a^3 + ... + n x a^n
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ay = a x (a + 2 x a^2 + 3 x a^3 + ... + n x a^n)
ay = a^2 + 2 x a^3 + 3 x a^4 + ... + (n - 1) x a^n + n x a^(n + 1)
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y - ay = (a + 2 x a^2 + 3 x a^3 + ... + n x a^n ) - (a^2 + 2 x a^3 + 3 x a^4 + ... + (n - 1) x a^n + n x a^(n + 1))
y - ay = a + (2 x a^2 - a^2) + (3 x a^3 - 2 x a^3) + ... + (n x a^n - (n - 1) x a^n) - n x a^(n + 1)
y (1 - a) = (a + a^2 + a^3 + ... + a^n) - n x a^(n + 1)
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a + a^2 + a^3 + ... + a^n
Let
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z = a + a^2 + a^3 + ... + a^n
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z / a = 1 + a + a^2 + ... + a^(n - 1)
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z - (z / a) = a + a^2 + a^3 + ... + a^n - (1 + a + a^2 + ... + a^(n - 1)) = a^n - 1
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z(1 - 1/a) = a^n -1