Lesson 10 of 15
Putting a number in to find out
Show on a big screenSix lines are on a board. Some of them are wrong. How can you find which?
x = 4 into both forms. Step 1 of 4
- Put
x = 4into both forms. (x + 3)(x + 5)→ 63.x² + 8x + 15→ 63. Match.- Shortcut
x² + 15→ 31. No match.
Put the same number into the old line and into each rewrite. A rewrite is a new form of the old line. A different answer shows that rewrite is wrong.
Rule: put a value into both forms and compare. One mismatch proves the rewrite wrong.
Two lines that are equal give the same number at every value.
A match proves less than a mismatch. At x = 0, (x + 3)(x + 5) and the wrong shortcut both give 15. They match at this one value, by chance.
Slide x. Look for a mismatch. Try each rewrite.
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Try the lab to finish this lesson.