A. Maths Q about Completing square
2007-11-06 2:28 am
The maximum value of the function f(x)=4k+18x-kx^2(k is a positive constant) is 45.Find k
回答 (1)
2007-11-06 2:36 am
✔ 最佳答案
F(x)=4k+18x﹣kx2 = -kx2+18x+4k
= -k(x2﹣18x/k)+4k
= -k(x2﹣18x/k + 81/k2)+4k+k(81/k2)
= -k(x﹣9/k)2+4k + 81/k
∵The maximum value of the function is 45
∴4k+81/k=45
4k2﹣45k+81=0
(4k﹣9)(k﹣9)=0
k=9/4 or 9
收錄日期: 2021-04-26 17:49:49
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https://hk.answers.yahoo.com/question/index?qid=20071105000051KK08310