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Intern  Joined: 18 Nov 2011
Posts: 33
Concentration: Strategy, Marketing
GMAT Date: 06-18-2013
GPA: 3.98
Adding exponents with the same base  [#permalink]

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I know the usual rules about multiplying exponents and dividing exponents, but I was always under the impression that ADDING exponents with the same base is not possible. For example, I thought it was not possible to simplify $$x^n + x^m$$. But then I saw this operation written and I have no idea how or why this works:

$$3^{17} - 3^{16} + 3^{15} = ?$$
$$= 3^{15}(9 - 3 + 1)$$
$$= 3^{15}(7)$$

And yes, they are equal Can someone explain how this method works, thanks

Edit: Check for more HERE.

Originally posted by hitman5532 on 15 Jan 2013, 16:18.
Last edited by Bunuel on 26 Feb 2019, 05:13, edited 1 time in total.
Updated.
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Re: Adding exponents with the same base  [#permalink]

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hitman5532 wrote:
I know the usual rules about multiplying exponents and dividing exponents, but I was always under the impression that ADDING exponents with the same base is not possible. For example, I thought it was not possible to simplify $$x^n + x^m$$. But then I saw this operation written and I have no idea how or why this works:

$$3^{17} - 3^{16} + 3^{15} = ?$$
$$= 3^{15}(9 - 3 + 1)$$
$$= 3^{15}(7)$$

And yes, they are equal Can someone explain how this method works, thanks

$$a + b + c = a(\frac{a}{a} + \frac{b}{a} + \frac{c}{a}) = a(1 + \frac{b}{a} + \frac{c}{a})$$

Similarly,

$$3^{17} - 3^{16} + 3^{15} = 3^{15}(\frac{3^{17}}{3^{15}} - \frac{3^{16}}{3^{15}} + \frac{3^{15}}{3^{15}})$$

$$= 3^{15}(3^2 - 3^1 + 1) = 3^{15}(9 - 3 + 1) = 3^{15}(7)$$
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Intern  Joined: 13 Mar 2011
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Re: Adding exponents with the same base  [#permalink]

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hitman5532 wrote:
I know the usual rules about multiplying exponents and dividing exponents, but I was always under the impression that ADDING exponents with the same base is not possible. For example, I thought it was not possible to simplify $$x^n + x^m$$. But then I saw this operation written and I have no idea how or why this works:

$$3^{17} - 3^{16} + 3^{15} = ?$$
$$= 3^{15}(9 - 3 + 1)$$
$$= 3^{15}(7)$$

And yes, they are equal Can someone explain how this method works, thanks

you are not working with exponents while solving the equation. the solution takes out the exponent part(3^{15) as it is common among all the values and then adds or subtracts the remaining numbers(9, -3, 1).

so all you do while solving such questions is that you find out an exponent-base pair which is common to all the numbers and take it out, leaving behind simple numbers to work with. makes sense??
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Re: Adding exponents with the same base  [#permalink]

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hitman5532 wrote:
I know the usual rules about multiplying exponents and dividing exponents, but I was always under the impression that ADDING exponents with the same base is not possible. For example, I thought it was not possible to simplify $$x^n + x^m$$. But then I saw this operation written and I have no idea how or why this works:

$$3^{17} - 3^{16} + 3^{15} = ?$$
$$= 3^{15}(9 - 3 + 1)$$
$$= 3^{15}(7)$$

And yes, they are equal Can someone explain how this method works, thanks

Check out this post on how to take common terms when dealing with exponents:

http://www.veritasprep.com/blog/2011/07 ... s-applied/
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Re: Adding exponents with the same base  [#permalink]

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hitman,

There are no general rules for addition or subtraction of exponents(unlike multiplication and addition), instead on the GMAT almost all such problems are simplified using the distributive property of multiplication over addition:
$$a(b+c) = ab + ac$$

For example:
$$42 + 56 = (7)(6) + (7)(8) = 7(6+8) = 7(14) = 98$$

We use the same idea for exponent terms:
$$3^{11} - 3^{10} = (3^{10})(3) - 3^{10}(1) = 3^{10}(3 - 1) = 3^{10}(2)$$

Another example that I use with students:
$$2^{20} - 3(2^{18}) = 2^{18}(2^2 - 3) = 2^{18}(4 - 3) = 2^{18}$$

Here is a list of official GMAT problems that use the same general principle to solve these types of problems:
If you haven't taken the GMATPrep practice test, then skip the ones listed as Official GMATPrep.

1) Old GMAT Paper Test(Easy): http://gmatclub.com/forum/5-12-5-13-a-5 ... 49552.html
2) Official GMATPrep(Medium): http://gmatclub.com/forum/which-of-the- ... 60927.html
3) Official GMAT Test(Medium): http://gmatclub.com/forum/what-is-the-g ... 70126.html
4) Official GMAT Test(Hard): http://gmatclub.com/forum/if-5-x-5-x-3- ... 09080.html
5) Official Guide GMAT 13th Edition(Hard): http://gmatclub.com/forum/the-value-of- ... 30682.html
6) Official GMATPrep Software(Hard): http://gmatclub.com/forum/what-is-the-g ... 04757.html
7) Official GMATPrep(Hard): http://gmatclub.com/forum/if-3-x-3-x-1- ... 44795.html
8) Official GMAT Test(Medium): http://gmatclub.com/forum/if-2-x-2-x-2- ... fl=similar
9) Official GMATPrep(Hard): http://gmatclub.com/forum/if-2-x-2-x-2- ... 30109.html

I believe that is pretty much all of the Official GMAT questions on this subtopic, other than the ones on the real exam over the last few years.

Dabral
Intern  Joined: 18 Nov 2011
Posts: 33
Concentration: Strategy, Marketing
GMAT Date: 06-18-2013
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Re: Adding exponents with the same base  [#permalink]

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Thank you everyone for your replies. Seeing the multiple takes on methodology gave me a great feel for the concept. It was one of those 'so obvious I missed it' things.
Intern  Joined: 21 Nov 2018
Posts: 2
Re: Adding exponents with the same base  [#permalink]

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So this is how it works. You first have 3^17−3^16+3^15=?
then you get
=3^15(9−3+1) for, you have to get a common factor
then you get 3^15(7)
i you care to simplify.
what about a kodu? eh? or 2. or 3. or 1000000000000000000000000000000000000000000000000000000000000000000
If you like this post, tell me through the kodu button Math Expert V
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Re: Adding exponents with the same base  [#permalink]

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hitman5532 wrote:
I know the usual rules about multiplying exponents and dividing exponents, but I was always under the impression that ADDING exponents with the same base is not possible. For example, I thought it was not possible to simplify $$x^n + x^m$$. But then I saw this operation written and I have no idea how or why this works:

$$3^{17} - 3^{16} + 3^{15} = ?$$
$$= 3^{15}(9 - 3 + 1)$$
$$= 3^{15}(7)$$

And yes, they are equal Can someone explain how this method works, thanks

EXPONENTS

Exponents are a "shortcut" method of showing a number that was multiplied by itself several times. For instance, number $$a$$ multiplied $$n$$ times can be written as $$a^n$$, where $$a$$ represents the base, the number that is multiplied by itself $$n$$ times and $$n$$ represents the exponent. The exponent indicates how many times to multiple the base, $$a$$, by itself.

Exponents one and zero:
$$a^0=1$$ Any nonzero number to the power of 0 is 1.
For example: $$5^0=1$$ and $$(-3)^0=1$$
• Note: the case of 0^0 is not tested on the GMAT.

$$a^1=a$$ Any number to the power 1 is itself.

Powers of zero:
If the exponent is positive, the power of zero is zero: $$0^n = 0$$, where $$n > 0$$.

If the exponent is negative, the power of zero ($$0^n$$, where $$n < 0$$) is undefined, because division by zero is implied.

Powers of one:
$$1^n=1$$ The integer powers of one are one.

Negative powers:
$$a^{-n}=\frac{1}{a^n}$$

Powers of minus one:
If n is an even integer, then $$(-1)^n=1$$.

If n is an odd integer, then $$(-1)^n =-1$$.

Operations involving the same exponents:
Keep the exponent, multiply or divide the bases
$$a^n*b^n=(ab)^n$$

$$\frac{a^n}{b^n}=(\frac{a}{b})^n$$

$$(a^m)^n=a^{mn}$$

$$a^m^n=a^{(m^n)}$$ and not $$(a^m)^n$$ (if exponentiation is indicated by stacked symbols, the rule is to work from the top down)

Operations involving the same bases:
Keep the base, add or subtract the exponent (add for multiplication, subtract for division)
$$a^n*a^m=a^{n+m}$$

$$\frac{a^n}{a^m}=a^{n-m}$$

Fraction as power:
$$a^{\frac{1}{n}}=\sqrt[n]{a}$$

$$a^{\frac{m}{n}}=\sqrt[n]{a^m}$$

ROOTS

Roots (or radicals) are the "opposite" operation of applying exponents. For instance x^2=16 and square root of 16=4.

General rules:
• $$\sqrt{x}\sqrt{y}=\sqrt{xy}$$ and $$\frac{\sqrt{x}}{\sqrt{y}}=\sqrt{\frac{x}{y}}$$.

• $$(\sqrt{x})^n=\sqrt{x^n}$$

• $$x^{\frac{1}{n}}=\sqrt[n]{x}$$

• $$x^{\frac{n}{m}}=\sqrt[m]{x^n}$$

• $${\sqrt{a}}+{\sqrt{b}}\neq{\sqrt{a+b}}$$

• $$\sqrt{x^2}=|x|$$, when $$x\leq{0}$$, then $$\sqrt{x^2}=-x$$ and when $$x\geq{0}$$, then $$\sqrt{x^2}=x$$

• When the GMAT provides the square root sign for an even root, such as $$\sqrt{x}$$ or $$\sqrt{x}$$, then the only accepted answer is the positive root.

That is, $$\sqrt{25}=5$$, NOT +5 or -5. In contrast, the equation $$x^2=25$$ has TWO solutions, +5 and -5. Even roots have only a positive value on the GMAT.

• Odd roots will have the same sign as the base of the root. For example, $$\sqrt{125} =5$$ and $$\sqrt{-64} =-4$$.

8. Exponents and Roots of Numbers

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GMAT 1: 790 Q51 V49 GRE 1: Q170 V170 Re: Adding exponents with the same base  [#permalink]

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You've already gotten some good answers, but I want to link this article as well for any curious readers who might find this thread! https://www.manhattanprep.com/gre/blog/ ... exponents/ It's on our GRE blog, but all of the math there should apply to the GMAT as well.
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My latest GMAT blog posts | Suggestions for blog articles are always welcome! Re: Adding exponents with the same base   [#permalink] 28 Feb 2019, 16:47
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