推 talrusha : 感謝幫忙!! 10/25 13:44
※ 引述《talrusha (黑桃Ace)》之銘言:
: 如圖,讀國中妹妹碰到的題目,小魯不才數學都還老師了@@
: https://imgur.com/RIiHJI2
: 已知矩形邊長 AB為15 AD為8 ,且紅色區域面積為9
: 求三角形 BPD面積
: 感謝各位大大相助!!
AP交DB於M,AC交PB於K,AC交BD於O
DP = x
DM : MB = x : 15 => DM : MO = 2x : (15 - x)
PK : KB = (15 - x) : 15
△BOP = 2(15 - x) + 30 - 4(15 - x) = 2x
9 = 2x * (15 - x)/(x + 15) + 2x * [(15 - x)/(30 - x)]
=> 9(x + 15)(x - 30) = -2x(x - 15)(x - 30) + 2x(x - 15)(x + 15)
=> (x + 15)(x - 30) = 10x(x - 15)
=> x^2 - 15x + 50 = 0
=> x = 5, 10
題目沒說DP < PC
所以兩解都應該取
x = 5:
△BPD = (1/2) * 5 * 8 = 20
x = 10:
△BPD = (1/2) * 10 * 8 = 40
註:兩個答案加起來要為8 * 15 / 2 = 60
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