At a canyon training station, Emek and Tara faced three marked platforms above a deep gap. The route instructions said to choose the platform with the smallest positive value. The labels were |3 - sqrt(20)|, sqrt(5)/2, and 9/8. A display beside the first platform showed sqrt(20) as approximately 4, making its value appear to be |3 - 4| = 1. That seemed like the obvious choice, but Tara noticed that the approximation was too rough. They simplified sqrt(20) to 2sqrt(5), so the first value was 2sqrt(5) - 3, approximately 1.472. The second value was approximately 1.118, while 9/8 equaled 1.125. Although the last two values were close, Emek confirmed that sqrt(5)/2 was smaller by comparing their exact forms. They rejected the tempting first route and stepped onto the platform marked sqrt(5)/2. The remaining signals turned green, guiding them safely across the canyon.

Challenges

Challenge 1 of 5

Challenges

Challenge 1 of 4
Hint
Compare the rough estimate of the first value with the more accurate value found after simplifying sqrt(20).
The display estimates sqrt(20) as 4 and therefore estimates |3 - sqrt(20)| as 1. Why is this estimate not reliable enough for choosing the smallest platform value?
Absolute value can be used only with rational numbers.The estimate changes |3 - sqrt(20)| to 1, even though its more accurate value is about 1.472, which can change the comparison.sqrt(20) is exactly 4, but absolute value changes 1 into -1.The estimate is unreliable because sqrt(20) is less than 3.
Correct. Rounding sqrt(20) too roughly makes the first platform appear smaller than it really is.
Check how replacing sqrt(20) by 4 changes the value of the entire absolute-value expression.
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Hint
Square both the coefficient 4 and sqrt(5).
To compare sqrt(5)/2 with 9/8 exactly, Emek multiplies both positive values by 8, giving 4sqrt(5) and 9. He then squares both positive quantities. What integer is the square of 4sqrt(5)?
Correct. Squaring 4sqrt(5) gives 16 times 5, or 80.
Use (4sqrt(5))^2 = 4^2 times (sqrt(5))^2.
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Hint
Transform both positive values in the same way so their order is preserved, then compare their squares.
A student claims that 9/8 is smaller than sqrt(5)/2 because 9 is less than 5 times 2. Which exact argument correctly checks the two platform values?
Since 9 is greater than 8, 9/8 must be greater than every value containing sqrt(5).Since 5 is less than 9, sqrt(5)/2 must be greater than 9/8.Multiply both values by 8 to compare 4sqrt(5) and 9. Their squares are 80 and 81, so 4sqrt(5) is smaller and therefore sqrt(5)/2 is smaller than 9/8.Square only sqrt(5)/2 to get 5/4, and compare 5 with 9.
Correct. The exact comparison shows that sqrt(5)/2 is slightly smaller than 9/8.
Apply the same operations to both positive values and compare the resulting quantities.
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Sayı Kanyonunu Geçmek
TR
ARC_G11_NUM_02_STORY_03
Sayı Kanyonunu Geçmek
Crossing the Number Canyon
EN
ARC_G11_NUM_02_STORY_03
Crossing the Number Canyon