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