數學......(未知數) 【20點唷】

2008-03-22 2:04 am
1.某次數學測驗,試題共25題,答對一題得4分,答錯一題不但不給分還要到扣一分,沒有作答不計分,某生作答21題得74分,問該生答對多少題?

2.鉛筆66枝分給男生9人,女生7人,男生每人比女生每人多2枝,問男生、女生每人各得幾枝?

3.父、子現在年齡差是28歲,3年後父年是子年的3倍,問傅、子現年為多少?



用X.Y 解 麻煩囉^^!


贈上20點~~~

回答 (4)

2008-03-22 2:31 am
✔ 最佳答案
1. 設該學生答對x題,答錯y題.
 4x-y=74......(1)
 x+y=21......(2)
(1)+(2), 5x=95
     x=19
∴該學生答對19題.

2. 設男生和女生每人分別得x和y枝鉛筆.
 9x+7y=66......(1)
 x-y=2......(2)
(2), x=y+2......(3)
代 (3)入 (1), 9(y+2) + 7y = 66
      9y+18+7y = 66
      16y = 48
      y=3
      x=5
∴男生每人得5枝, 女生每人得3枝.

3. 設父現年x歲, 子現年y歲.
 x-y=28......(1)
 x+3 = 3(y+3)......(2)
(1), x=28+y......(3)
代(3)入(2), 28+y + 3 = 3(y+3)
      31+y = 3y+9
      2y = 22
      y = 11
      x = 39
∴父現年39歲, 子現年11歲.
2008-03-22 4:31 am
1. 設該學生答對x題,答錯y題.
 4x-y=74......(1)
 x+y=21......(2)
(1)+(2), 5x=95
     x=19
∴該學生答對19題.

2. 設男生和女生每人分別得x和y枝鉛筆.
 9x+7y=66......(1)
 x-y=2......(2)
(2), x=y+2......(3)
代 (3)入 (1), 9(y+2) + 7y = 66
      9y+18+7y = 66
      16y = 48
      y=3
      x=5
∴男生每人得5枝, 女生每人得3枝.

3. 設父現年x歲, 子現年y歲.
 x-y=28......(1)
 x+3 = 3(y+3)......(2)
(1), x=28+y......(3)
代(3)入(2), 28+y + 3 = 3(y+3)
      31+y = 3y+9
      2y = 22
      y = 11
      x = 39

2008-03-21 20:31:20 補充:
所以孩子現今年齡為11歲,父親為 11 + 28 = 39歲
2008-03-22 3:19 am
2)boy x girl y
9x+7y=66 x-2=y
9x+7(x-2)=66 5-2=y
x=5 y=3

3)son x father y
y-28=x (x+3)=y+3
(y-28+3)3=y+3 y-28=x
3y-75=y+3 39-28=x
y=39 x=11
2008-03-22 2:38 am
1)設x為答對題目,y為答錯題目
x + y = 21...a
4x - y = 74...b

x = 21-y...c
將c代入b
4(21-y) - 5 =74
-5y = -10
y = 2
x + 2 = 21
x = 19
所以該生答對19條題目

2)設女生可得 x 枝筆,男生 x+2 枝筆
7x + 9(x+2) = 66
16x = 48
x = 3
所以女生可得3枝筆,男生可得 3+2=5枝筆

3)設x為父親年齡,y為孩子年齡
x - y = 28...a
x + 3 = 3(y+3)...b

x = 28+y..c
將c代入b
28 + y +3 = 3y +9
31 - 9 = 2y
22 = 2y
y = 11
所以孩子現今年齡為11歲,父親為 11 + 28 = 39歲


收錄日期: 2021-04-25 13:16:45
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