# Dumb Question: Why Do You Add Exponents When Multiplying?

1. Nov 2, 2015

### pandaexpress

When multiplying x2 and x5, we get x7. Quick, dumb question:

2. Nov 2, 2015

### Staff: Mentor

Think of the exponent as counting the number of factors so x^2 is x*x and x^5 is x*x*x*x*x and when multiplied together gives you 7 x's or x^7

3. Nov 2, 2015

### Staff: Mentor

$x^2$ means $x \cdot x$, and $x^2$ means $x \cdot x \cdot x \cdot x \cdot x$, so $x^2 \cdot x^5$ means $x \cdot x \cdot x \cdot x \cdot x \cdot x \cdot x$, right? How many factors of x are there in that last expression?

4. Nov 2, 2015

### pandaexpress

This might seem a little dumb, but I figured that multiplying would give you 10 still. Using your example above,

x2 = x*x
x5 = x*x*x*x*x

Wouldn't x2 * x5 = x*x; x*x; x*x; x*x; x*x? Basically, you'd have a pair of x*x's five times. Or, you would get:

x*x*x*x*x; x*x*x*x*x Or, in other words, two groups of x*x*x*x*x.

It's the same idea with regular multiplication, where if you have 2X5, then it's like

|| || || || || (five groups of two) or |||||; ||||| (two groups of five)

I just don't quite understand why we're adding and getting 7 (instead of 10), since the operation is to multiply exponents. Do you guys see what I mean?

If they want you to add the exponents, then why not just say x2 + x5?

5. Nov 2, 2015

### Staff: Mentor

No.
What you have written is $(x^2)^5$, or 5 factors of $x^2$, making 10 factors of x, or $x^{10}$.
With $(x^2) \cdot (x^5)$ you have two factors of x multiplying 5 factors of x, making 7 factors of x, or $x^7$.
No, again. The operation is to multiply $x^2$ and $x^5$, which results in the exponents being added.
Because the terms in $x^2 + x^5$ are not like terms (i.e., same exponent), they can't be combined.

6. Nov 2, 2015

### DrewD

Maybe try looking at it with numbers. For example, when $x=2$, $x^2\cdot x^5$ is $4\cdot32=128=2^7$. Try it with some more numbers. Now you will see that indeed, as Mark44 said, $x^2\cdot x^5=(x\cdot x)\cdot(x\cdot x\cdot x\cdot x\cdot x)$ and not what you wrote.