18 times 5 is 90, but what is the answer if both 18 and 5 are negative numbers? Here we will determine if the answer to negative 18 times negative 5 is positive 90 or negative 90.
There are many different ways to explain and determine if the answer to negative 18 times negative 5 is -90 or +90 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 -90 is the correct answer using basic algebra.
We will test if -90 is correct by setting up the equation negative 18 times negative 5 equals negative 90, and then simplifying the equation to see if it is true.
(-18) (-5) = (-90)
Divide both sides of the equation by (-5) :
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| = |
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The numerator and denominator on the left side both have (-5) which even each other out and can be removed:
| (-18) = |
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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:
| (-18) = |
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A numerator of 90 and denominator of 5 on the right side is the same as 90 divided by 5 which is 18. Now we end up with this untrue equation:
(-18) = (18)
-18 is NOT equal to 18, thus we have proven that negative 18 times negative 5 is not equal to -90. The answer must therefore be 90:
-18 x -5 = 90
The above applies to all math problems where you multiply two negative numbers. Multiplying two negative numbers such as negative 18 times negative 5 will always have a positive answer.
Negative Times Negative Calculator
Here you can multiply another two negative numbers together.
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