A solid circular cylinder has a base radius of r cm and a height of h cm. The cylinder has a volume of 1200π cm³ and a total surface area of S cm².
- (i)Show that S = 2πr² + 2400π/r.[3]
- (ii)Given that h and r can vary, find the stationary value of S and determine its nature.[5]
Mark scheme
Marking points from the official mark scheme (Oct/Nov 2019). The mark or mark type is shown in brackets.
- [1] B1 πr²h = 1200π
- [1] B1 h = 1200/r² or πrh = 1200π/r and substitution into their S
- [1] B1 S = 2πr² + (2πr * 1200/r²) leading to given answer
- [1] M1 Must obtain the form Ar + B/r²
- [1] M1 Dep for equating to zero and attempt to solve to obtain r = ...
- [1] A1 For correct r
- [1] A1 Smin = 1340 or 1341
- [1] B1 For a correct method to reach a correct conclusion
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