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## Let’s play a game!

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**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 = s3 s = 3V We say that V is the cube of s and say that s is the cube root of V**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:**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**Now a (much) more difficult question...**What is ? Try to guess!**First estimate**How big is ? 274625 The greater the side s,the greater the volume V 729 s=9 s=? <**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**Call a the decimal digit, call b the unit digit.**Then A preliminary result We know has two digits. In other words:**Find a, b!**Solve the equation: 274625 = 10 a b**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**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:**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**Equivalently:**Divide by 1000 and find a cube close to Answer: Hence:**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**In other words:**by Divide and find the quotient Conclusion: Right guess?**Let’s check!**Right!**And now…**try again! Good luck!