quadratic equation 問題 x 2,30分架

用戶228162

2010-08-22 10:37 pm

有2條quadratic equation ge問題想問下大家:

(我要詳細中文解釋同詳細ge steps! 重有要不時番黎睇下有冇補充)

1.A rectangular lawn 25 m by 20 m is surrounded by a path of uniform width. If
the area of the path is 306m^2(^2姐係2次方), find the width of the path.

2.If 2 more files can be bought for $120 when the price is lowered by $3 per each,
what is the original price per each?

THX!

更新1:

to:☂雨後陽光☀ 我要詳細中文解釋同詳細ge steps! 重有要不時番黎睇下有冇補充<----- thx!

回答 (1)

用戶2053

2010-08-23 12:53 am

✔ 最佳答案

1)Let w be the width :(2w + 25)(2w + 20) - 25*20 = 3064w^2 + 50w + 40w + 25*20 - 25*20 = 3064w^2 + 90w - 306 = 02w^2 + 45w - 153 = 0(2w + 51)(w - 3) = 0w = -51/2 (rejected) or w = 3The width of the path = 3 m
2)Let x be the original price per each :120 /(x - 3) - 120 / x = 2120x - 120(x - 3) = 2x(x - 3)360 = 2x^2 - 6xx^2 - 3x - 180 = 0(x - 15)(x + 12) = 0x = 15 or x = 12 (rejected)The original price is $15 per each.

2010-08-22 17:24:44 補充：
1)

設闊是w , 路徑連草地長 25+2w , 闊 20+2w ,

路徑連草地面積 - 草地面積 = 路徑面積

(2w + 25)(2w + 20) - 25*20 = 306

2010-08-22 17:28:12 補充：
2)設原價$x每個

原價時可買 120/x 個,
減3元時可買 120/(x-3) 個比原價時多買2個。

120 /(x - 3) - 120 / x = 2

2010-08-22 17:29:27 補充：
不如你開新題,5分都好

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