the cost of painting a solid cylinder of radius r cm and height h cm varied directly as its total surface area. if r=4cm and h=6cm, the cost is $20. find the cost when r=8 and h=14.
the ans. is 88
how can i get the answer?
plz step by step
thanks a lot!
varied directly math question
2012-08-31 9:12 pm
回答 (1)
2012-08-31 9:39 pm
✔ 最佳答案
Total surface area:2兀r²(the two circles)+2兀rh
Let $C be the cost.
C = k (2兀r²+2兀rh)
C = k·2兀r(r+h)
When r=4, h=6 and C=20,
20 = k·2兀(4)(4+6)
k=1/4兀
.·.C=r(r+h)/2
When r=8 and h=14,
C=8(8+14)/2
=88
.·.The cost is $88.
:)
2012-08-31 13:44:55 補充：
Sorry, I missed an important thing..
C = k (2兀r²+2兀rh), where k is a non-zero constant.
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