An infinite product...

Algebra Level 2

2 1 4 × 4 1 8 × 8 1 16 × \large 2^\frac 14 \times 4^\frac 18 \times 8^{\frac 1{16}} \times \cdots

What is the value of the product above?


The answer is 2.

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1 solution

Sumant Chopde
Apr 19, 2019

Let the value of this product be V. V = 2 1 / 4 × 2 2 / 8 × 2 3 / 16 \therefore V = 2^{1/4} \times 2^{2/8} \times 2^{3/16} \dots ∞

V = 2 1 / 4 + 2 / 8 + 3 / 16 + \therefore V = 2^{1/4 + 2/8 + 3/16 + \dots ∞}

Let V = 2 S V = 2^S

Now, S = 1 4 + 2 8 + 3 16 + S = \frac{1}{4} + \frac{2}{8} + \frac{3}{16} + \dots ∞

This is an arithemetico-geometric series.

We have, 1 2 ( S ) = 0 + 1 8 + 2 16 + 3 32 + \frac{1}{2} (S) = 0 + \frac{1}{8} + \frac{2}{16} + \frac{3}{32} + \dots ∞

Hence, we get S 1 / 2 ( S ) = ( 1 4 0 ) + ( 2 8 1 8 ) + ( 3 16 2 16 ) + S - 1/2 (S) = \big( \frac{1}{4} - 0 \big) + \big( \frac{2}{8} - \frac{1}{8} \big) + \big( \frac{3}{16} - \frac{2}{16} \big) + \dots ∞

1 / 2 ( S ) = 1 4 + 1 8 + 1 16 + \therefore 1/2 (S) = \frac{1}{4} + \frac{1}{8} + \frac{1}{16} + \dots ∞

This is an infinite geometric series with first term = 1 4 = \frac{1}{4} and common ratio = 1 2 = \frac{1}{2} .

Hence, 1 / 2 ( S ) = 1 / 4 1 1 2 1/2 (S) = \frac{1/4}{1-\frac{1}{2}}

That's why 1 / 2 ( S ) = 1 / 2 1/2 (S) = 1/2

Hence, S = 1 S = 1

And V = 2 S = 2 1 = 2 V = 2^S = 2^1 = \boxed{2}

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