Eli5: Why is 2^9 + 2^9 = 2^10

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Middle school teacher here, and this is driving me crazy! Thanks!

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14 Answers

Anonymous 0 Comments

This is a weird property of powers of 2, not a general property of exponents!

Think about what an exponent is. If we have `x^y`, that means we have `x*x` for as many times as `y` implies. So `2^y` is 2 times itself that many times.

2^1 = 2
2^2 = 2 * 2 = 2 + 2 = 4
2^3 = 4 * 2 = 4 + 4 = (2 + 2) + (2 + 2) = 2^2 + 2^2 = 8
2^4 = 2^3 * 2 = 2^3 + 2^3 = 8 + 8 = 16

It doesn’t work for other numbers.

3^1 = 3
3^2 = 3 * 3 = 9
But 3 + 3 = 6 != 9. Oops!
3^3 = 3 * 3 * 3 = 27
But 3^2 + 3^2 = 9 + 9 = 18, oops!

Basically it’s a fun trick because 2 * 2 and 2^2 are mathematically the same, which is not true for any other number.

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