Solving for Reciprocal: Calculating (7/8) to the Negative Power

Negative Exponents with Fractional Bases

(78)2=? (\frac{7}{8})^{-2}=\text{?}

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Step-by-step video solution

Watch the teacher solve the problem with clear explanations
00:00 Simplify the following problem
00:03 In order to eliminate a negative exponent
00:08 We'll flip the numerator and the denominator so that the exponent will become positive
00:12 We'll apply this formula to our exercise
00:17 When raising a fraction to a power, both the numerator and the denominator are raised to that power
00:24 We'll apply this formula to our exercise
00:32 We'll break down each exponent and then proceed to solve
00:40 This is the solution

Step-by-step written solution

Follow each step carefully to understand the complete solution
1

Understand the problem

(78)2=? (\frac{7}{8})^{-2}=\text{?}

2

Step-by-step solution

To solve the problem of evaluating (78)2(\frac{7}{8})^{-2}, we'll proceed with these steps:

  • Step 1: Convert the negative exponent into a positive exponent by taking the reciprocal of the base.
  • Step 2: Evaluate the expression obtained after the conversion.

Now, let's work through each step:

Step 1: Convert the negative exponent to a positive exponent using the reciprocal:
Using the property (ab)n=(ba)n(\frac{a}{b})^{-n} = (\frac{b}{a})^{n}, we have:

(78)2=(87)2 (\frac{7}{8})^{-2} = (\frac{8}{7})^{2}

Step 2: Calculate the positive power:

(87)2=8272=6449(\frac{8}{7})^{2} = \frac{8^2}{7^2} = \frac{64}{49}

Thus, the solution to the problem is:
(78)2=6449 (\frac{7}{8})^{-2} = \frac{64}{49}

Extra step to express 6449\frac{64}{49} as a mixed number:

64÷49=1 64 \div 49 = 1 remainder 1515, so 6449=11549\frac{64}{49} = 1 \frac{15}{49} .

Therefore, the simplified solution to the expression (78)2(\frac{7}{8})^{-2} is 11549 1 \frac{15}{49} .

3

Final Answer

11549 1\frac{15}{49}

Key Points to Remember

Essential concepts to master this topic
  • Rule: Negative exponent means take reciprocal then apply positive exponent
  • Technique: (78)2=(87)2=6449 (\frac{7}{8})^{-2} = (\frac{8}{7})^{2} = \frac{64}{49}
  • Check: Convert to mixed number: 64 ÷ 49 = 1 remainder 15 ✓

Common Mistakes

Avoid these frequent errors
  • Making the entire expression negative
    Don't think (78)2 (\frac{7}{8})^{-2} equals 4964 -\frac{49}{64} ! The negative sign is in the exponent, not the result. Always flip the fraction first, then apply the positive exponent.

Practice Quiz

Test your knowledge with interactive questions

\( 112^0=\text{?} \)

FAQ

Everything you need to know about this question

Why doesn't the negative exponent make the answer negative?

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The negative exponent tells you to flip the fraction, not make it negative! Think of it as: an=1an a^{-n} = \frac{1}{a^n} . The result can still be positive.

How do I remember to flip the fraction?

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Use this memory trick: "Negative exponent = Reciprocal". So (78)2 (\frac{7}{8})^{-2} becomes (87)2 (\frac{8}{7})^{2} - just flip and make the exponent positive!

Do I need to convert to a mixed number?

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It depends on what the question asks for! 6449 \frac{64}{49} and 11549 1\frac{15}{49} are the same value, just in different forms. Check your answer choices to see which format is expected.

What if the base was a whole number instead of a fraction?

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Same rule applies! For example: 32=132=19 3^{-2} = \frac{1}{3^2} = \frac{1}{9} . The negative exponent always means "take the reciprocal".

Can I use a calculator for this?

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Yes, but be careful with parentheses! Enter it as (7/8)^(-2). However, understanding the steps helps you check if your calculator answer makes sense.

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