81 times 9 is 729, but what is the answer if both 81 and 9 are negative numbers? Here we will determine if the answer to negative 81 times negative 9 is positive 729 or negative 729.
There are many different ways to explain and determine if the answer to negative 81 times negative 9 is -729 or +729 using mathematical terms such as additive inverse and mathematical laws like the Distribute Law.
We will take a different approach - process of elimination: We will simply test if -729 is the correct answer using basic algebra.
We will test if -729 is correct by setting up the equation negative 81 times negative 9 equals negative 729, and then simplifying the equation to see if it is true.
(-81) (-9) = (-729)
Divide both sides of the equation by (-9) :
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| = |
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The numerator and denominator on the left side both have (-9) which even each other out and can be removed:
| (-81) = |
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The numerator and denominator on the right side both have a minus sign (-) which even each other out and can be removed:
| (-81) = |
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A numerator of 729 and denominator of 9 on the right side is the same as 729 divided by 9 which is 81. Now we end up with this untrue equation:
(-81) = (81)
-81 is NOT equal to 81, thus we have proven that negative 81 times negative 9 is not equal to -729. The answer must therefore be 729:
-81 x -9 = 729
The above applies to all math problems where you multiply two negative numbers. Multiplying two negative numbers such as negative 81 times negative 9 will always have a positive answer.
Negative Times Negative Calculator
Here you can multiply another two negative numbers together.
What is negative 81 times negative 10?
Here is the next problem on our list that we have explained and calculated.
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