maths LINEAR INEQUALITIES IN ONE UNKNOWN F.3數

crystal

2006-12-22 12:58 am

Mike 想買手電 and choose a service provider.If he chooses A.T., 佢可以買1手電 with$2880 and 月費係 $168.A.T. will offer to him a rebate of $100per month for 12 months.If he chooses U.T.,佢可以買1手電with$3680 and月費係$128.U. T. will offer to him a rebate of $80 per month for 15 months. Suppose the mobile phone and the service provided by both companies are the same. Mike chooses U.T. at least how months does Mike joins the service provided by U.T. so total amount he pays will less than charged by A.T.?
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回答 (1)

myisland8132

2006-12-22 1:12 am

✔ 最佳答案

if Mike choose AT
then the charge is
2880+(168-100)t =2880+68t (if 1<= t<=12)
2880+68t +168(t-12) (if t>12)
if Mike choose UT
then the charge is
3680+(128-80)t =3680+48t (if 1<= t<=15)
3680+48t +128(t-15) (if t>15)
when t=12
the charge of AT is 3696
the charge of UT is 4256
when t=15
the charge of AT is 4404
the charge of UT is 4400
at least 15 months Mike joins the service provided by U.T. th3 total amount he pays will less than charged by A.T.

2006-12-21 17:13:54 補充：
when t=13, 14, we can show that the charge of AT is still less than UT

2006-12-21 17:23:07 補充：
做錯the charge of AT is3696+168(t-12) (if t>12)the charge of UT is4400 +128(t-15) (if t>15)so when t=15the charge of AT is 4200the charge of UT is 4400let4400 +128(t-15)<= 3696+168(t-12) 704+128t-1920<=168t-2016800<=40tt>=20at least 20 months

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