The y-axis is on a logarithmic scale, so this value that we have deduced for ΔY represents the logarithm of the change in the amount of 131I that has occurred during 25 days. Using the general rules we established in Box 7, we know that because we are multiplying a positive number by a negative number the answer will be a negative number, thus, ΔY = −0.0866 × 25 = −2.165 These rules will come in handy as you work through the following example question.
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The general idea is that if you multiply (or divide) a positive number by a negative number then that changes the sign of your eventual answer or, put another way: multiplying or dividing the same signs together results in a positive number, whilst doing the same with opposite signs give a negative number. The rule is that a positive number multiplied by a negative number (or the other way round), gives a negative number, so 2 × −2 = −4 (and −2 × 2 = −4). However, what do you get if you multiply 2 × −2? If you multiply 2 × 2, you get 4, so, a positive number multiplied by a positive number, gives a positive number. Multiplication and division involving negative numbers Now here's the tricky one: subtracting a negative number is the same as adding a positive number, so:Īnother helpful way to view the same calculation is as follows: Using the thermometer scale above, you should be able to verify for yourself that the following sums are correct: Using a scale like this we can come up with some general rules for addition and subtraction involving negative numbers.Īddition and subtraction involving negative numbers
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The temperature difference between −1 ☌ and +1 ☌ is 2 ☌. Similarly, the temperature difference between 0 ☌ and −1 ☌ is also 1 ☌. The temperature difference between 0 ☌ and 1 ☌ is 1 ☌. As temperatures get colder and colder below 0 ☌ their values also increase, but in negative numbers. Temperatures above freezing go up incrementally in positive numbers. The freezing point of water is marked as 0 ☌.
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A negative number is any value less than zero.