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Let’s play a game!

Let’s play a game! . }. =?. =?. =?. Guess cube roots. Recall: What is a Cube Root?. The edges of a cube all have the same measure: let’s call it s . s. The volume V of this cube is then given by the formula. s. s. We can also write. V = s 3. s = 3 V. We say that

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Let’s play a game!

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  1. Let’s play a game! } =? =? =? Guess cube roots

  2. Recall: What is a Cube Root? • The edges of a cube all have the same measure: let’s call it s. s • The volume V of this cube is then given by the formula s s We can also write V = s3 s = 3V We say that V is the cube of s and say that s is the cube root of V

  3. Some Easy Arithmetic First compute the volumes V of cubes with given sides s s 1 2 3 4 5 6 7 8 9 V 1 8 27 64 125 216 343 512 729 What is the cube root of 343? And now answer:

  4. Answer! • The cube root of 343 is 7, i.e., V = 343 In other words: • The side of a cube of volume V = 343 is s = 7. s = 7

  5. Now a (much) more difficult question... What is ? Try to guess!

  6. First estimate How big is ? 274625 The greater the side s,the greater the volume V 729 s=9 s=? <

  7. Second Estimate How many digits has ? Since it is greater than 9, it has at least two. On the other hand: Compute 993 = 970299. Compare 970299 with 274625. Since 970299 >274625, we have that

  8. Call a the decimal digit, call b the unit digit. Then A preliminary result We know has two digits. In other words:

  9. Find a, b! Solve the equation: 274625 = 10 a b

  10. Expand the Cubic Expression 10a*10a*b = 100a2b 10a*b *b = 10ab2 10a*b*b = 10ab2 b* b*b =b3 10a*10a*b= 100a2b 10a * 10a *b= 100a2b 10a*10a* 10a= 1000a3 10a * b* b= 10ab2

  11. 3* 2 * 3 3* * 2 3 Summing up! 100a2b 100a2b 1000a3 10ab2 10ab2 100a2b 10ab2 b3 1000a3 3*100a2b 3*10ab2 b3 Compare with the formula for cubic expressions:

  12. 10a*10a*b = 100a2b b* b *10a = 10ab2 10a*b*b = 10ab2 b* b*b =b3 10a*10a*b= 100a2b b* 10a * 10a= 100a2b (10a)3= 1000a3 b* 10a * b= 10ab2 274625 Step 1: is a perfect cube close to 1000a3

  13. Equivalently: Divide by 1000 and find a cube close to Answer: Hence:

  14. 100a2b 10a*10a*b = 100a2b b* b *10a = 10ab2 10a*b*b = 10ab2 b* b*b =b3 100a2b 10a*10a*b= 100a2b b* 10a * 10a= 100a2b 100a2b b* 10a * b= 10ab2 58625 Step 2: from Subtract . and obtain Approximate it with

  15. In other words: by Divide and find the quotient Conclusion: Right guess?

  16. Let’s check! Right!

  17. And now… try again! Good luck!

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