8 times 90 is 720, but what is the answer if both 8 and 90 are negative numbers? Here we will determine if the answer to negative 8 times negative 90 is positive 720 or negative 720.
There are many different ways to explain and determine if the answer to negative 8 times negative 90 is -720 or +720 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 -720 is the correct answer using basic algebra.
We will test if -720 is correct by setting up the equation negative 8 times negative 90 equals negative 720, and then simplifying the equation to see if it is true.
(-8) (-90) = (-720)
Divide both sides of the equation by (-90) :
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| = |
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The numerator and denominator on the left side both have (-90) which even each other out and can be removed:
| (-8) = |
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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:
| (-8) = |
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A numerator of 720 and denominator of 90 on the right side is the same as 720 divided by 90 which is 8. Now we end up with this untrue equation:
(-8) = (8)
-8 is NOT equal to 8, thus we have proven that negative 8 times negative 90 is not equal to -720. The answer must therefore be 720:
-8 x -90 = 720
The above applies to all math problems where you multiply two negative numbers. Multiplying two negative numbers such as negative 8 times negative 90 will always have a positive answer.
Negative Times Negative Calculator
Here you can multiply another two negative numbers together.
What is negative 8 times negative 91?
Here is the next problem on our list that we have explained and calculated.
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