Explain how to calculate 75 + 72 + 69 + 66 + ... + 21 + 18 + 15 + ... + (-69) + (-72) + (-75) + (-78) + (-81) without a calculator.
75 + 72 + 69 + 66 + ... + 3 + 0 + -3 ... + -78 + -81
If you notice, every single term on the left and right side cancel out except for -78 and -81
-78 - 81 = -(78 + 81) = -159
