C2 geometric question Watch

alesha98
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In question 6b, i formed an equation ar^(n-1) < 500. So i got n is 17 therefore should be in year 2016. But i am not sure why the answer in the mark scheme is year 2015? When i do it in a calculator, in year 2015, the value of the machine is still higher than 500?
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Paranoid_Glitch
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(Original post by alesha98)
In question 6b, i formed an equation ar^(n-1) < 500. So i got n is 17 therefore should be in year 2016. But i am not sure why the answer in the mark scheme is year 2015? When i do it in a calculator, in year 2015, the value of the machine is still higher than 500?
ar^n-1<500
15000*0.8^n-1<500

0.8^n-1<500/15000

(n-1)log0.8<log(1/30)

n-1>15.24219437

n>15.24219437+1
n>16.24219437

n=17?

Not sure if that is the answer, just the way i would work it out.
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alesha98
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(Original post by Paranoid_Glitch)
ar^n-1<500
15000*0.8^n-1<500

0.8^n-1<500/15000

(n-1)log0.8<log(1/30)

n-1>15.24219437

n>15.24219437+1
n>16.24219437

n=17?

Not sure if that is the answer, just the way i would work it out.
yeah i got n=17 too, so it should be in year 2016? but the answer on mark scheme is year 2015
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Paranoid_Glitch
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(Original post by alesha98)
yeah i got n=17 too, so it should be in year 2016? but the answer on mark scheme is year 2015
Because it isn't a geometric sequence? Haha makes no sense. Sorry
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ghostwalker
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(Original post by alesha98)
In question 6b, i formed an equation ar^(n-1) < 500. So i got n is 17 therefore should be in year 2016. But i am not sure why the answer in the mark scheme is year 2015? When i do it in a calculator, in year 2015, the value of the machine is still higher than 500?

Value of machine drops during the year, not suddenly at the end of the year.

At the start of 2016 its value was less than 500. At start of 2015 it was above 500. So, it dropped below 500 during 2015
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alesha98
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(Original post by ghostwalker)
Value of machine drops during the year, not suddenly at the end of the year.

At the start of 2016 its value was less than 500. At start of 2015 it was above 500. So, it dropped below 500 during 2015
the question said its value drops at the start of the year. So surely at the start of 2016, it drops below 500?
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ghostwalker
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(Original post by alesha98)
the question said its value drops at the start of the year. So surely at the start of 2016, it drops below 500?
In red - question doesn't say that. It say each year the value drops by 20% of its value at the start of the year. This does not imply a sudden drop after a year.
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alesha98
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(Original post by ghostwalker)
In red - question doesn't say that. It say each year the value drops by 20% of its value at the start of the year. This does not imply a sudden drop after a year.
i am still confused about it ... hopefully it wont come up in the c2 exam
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