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Opens on Chapter 4’s example: the overnight drop from 5 degrees to −8: 5 − (−8).
How it works
On the line, + means move right and − means move left; a negative number reverses the move, so taking away −8 is the same as adding 8. For × and ÷, same signs give a positive and different signs a negative; the pattern shows why: each step down in the first number changes the answer by the same amount, and the pattern carries straight on past zero. The gap between two numbers is the higher minus the lower, whichever side of zero they sit. A power belongs to what sits right before it, so −x² is −(x²) while (−x)² squares the negative.
Numbers can be typed as 1,450, $38, 0.40, 12%, 1/6 or 2 3/8. The algebra is done with exact fractions, so 1/3 stays 1/3; decimals are rounded half up, the way the book rounds. Everything stays in your browser: no sign-up, nothing stored or sent.