Explain how to calculate 75 + 72 + 69 + 66 + ... + 21 + 18 + 15 + ... + (-69) + (-72) + (-75) + (-78) + (-81) without a calculator.
+38 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!
