Lesson 3 of 15
A test that can only say no
Show on a big screenA list has hundreds of numbers. You cannot search each one for a side. Look only at the last digit, the one at the right end. A digit is one of 0 to 9.
Step 1 of 4
- Square every digit, 0 to 9.
- Their last digits: 0, 1, 4, 9, 6, 5, 6, 9, 4, 1.
- Only six digits ever appear: 0, 1, 4, 5, 6, 9.
- 72 ends in 2, so it is out. 26 ends in 6, so it still needs a hunt.
Rule: A square ends in 0, 1, 4, 5, 6 or 9. Ending in 2, 3, 7 or 8 rules a number out. Ending in 0, 1, 4, 5, 6 or 9 needs a check.
72 ends in 2, so 72 is out. 26 ends in 6, so it needs a check. 5 × 5 = 25 and 6 × 6 = 36. 26 is between them, so it is not a square.
A square also has an even number of zeros at its end. 400 has two. 4000 has three, so it is not a square.
Square a few sides. Then drag the number to the right shelf.
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Try the lab to finish this lesson.