Explain how to calculate 75 + 72 + 69 + 66 + ... + 21 + 18 + 15 + ... + (-66) + (-69) + (-72) + (-75) without a calculator.
+1622 Well... the parenthesis are pretty useless, if we remove them it turns from +(-66) to -66. The first and last terms of the expression are 75 and -75. The common difference is 3. So the middle term will be 0. Then we can deduce that every term cancels out, and then the answer is ZERO!!!
