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

 Nov 13, 2022
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Explain how to calculate 75 + 72 + 69 + 66 + ... + 21 + 18 + 15 + ... + (-69) + (-72) + (-75) + (-78) + (-81) without a calculator.  

 

How many times does the same question have to be answered?  

Here's a copy/paste of the answer I submitted a couple days ago:  

 

Line them up like this  

 

                      75 + 72 + 69 + 66 + ... + 9 + 6 + 3   

    –81 – 78 – 75 – 72 – 69 – 66 – ... – 9 – 6 – 3   

 

Get the idea?  All the numbers cancel out except those two on the end.  

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 Nov 13, 2022

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