78 times 21 is 1638, but what is the answer if both 78 and 21 are negative numbers? Here we will determine if the answer to negative 78 times negative 21 is positive 1638 or negative 1638.
There are many different ways to explain and determine if the answer to negative 78 times negative 21 is -1638 or +1638 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 -1638 is the correct answer using basic algebra.
We will test if -1638 is correct by setting up the equation negative 78 times negative 21 equals negative 1638, and then simplifying the equation to see if it is true.
(-78) (-21) = (-1638)
Divide both sides of the equation by (-21) :
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| = |
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The numerator and denominator on the left side both have (-21) which even each other out and can be removed:
| (-78) = |
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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:
| (-78) = |
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A numerator of 1638 and denominator of 21 on the right side is the same as 1638 divided by 21 which is 78. Now we end up with this untrue equation:
(-78) = (78)
-78 is NOT equal to 78, thus we have proven that negative 78 times negative 21 is not equal to -1638. The answer must therefore be 1638:
-78 x -21 = 1638
The above applies to all math problems where you multiply two negative numbers. Multiplying two negative numbers such as negative 78 times negative 21 will always have a positive answer.
Negative Times Negative Calculator
Here you can multiply another two negative numbers together.
What is negative 78 times negative 22?
Here is the next problem on our list that we have explained and calculated.
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