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Explain how to calculate 75 + 72 + 69 + 66 + ... + 21 + 18 + 15 + ... + (-69) + (-72) + (-75) + (-78) + (-81) without a calculator.

 Nov 6, 2022
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This can be easily calculated this way. 

 

on both sides of the equation, there is a positive and a corresponding negative number. 

 

(75 and -75), (72 and -72), etc

 

So what this does is it cancels out all numbers as adding 75 to -75 will equal 0

 

so all numbers will cancel out their corresponding negative except for the two negatives with no corresponding number (-78 and -81)

 

So what it will leave you with is something much more simple to solve

 

-78+(-81)

 

Hope this helps!

 Nov 7, 2022

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