請幫手數學問題!!!!!

2012-09-02 11:42 pm
題目
a)12x^2-16kx+5k^2=0 is a quadratic equation in x where k is an interger.It is given that 3/2 is a root of the equation.By using the quadratic formula,find
a)the value of k,
b)the other root of the equation.

答案
a) x = {16k + √[(16k)^2 - 4(12)(5k^2)]}/2(12)
or
x = {16k - √[(16k)^2 - 4(12)(5k^2)]}/2(12)
x = 5k/6 or x = k/2 <----------------請問呢步點出?
As k is an integer, k should be 3 <-------------------------------呢到又點解會係3?
(b) The other root is 5k/6 = 15/6 = 5/2<------------點解other root係5k/6而唔係k/2?

回答 (2)

2012-09-04 12:27 am
✔ 最佳答案
題目的意思是:
一元二次方程 12x² - 16kx + 5k² = 0,其中 k 為整數。已知 3/2 為方程式其中一根(即 x 可以是 3/2)。使用一元二次方程的公式,計算
a) k 之值。

解法是首先解一元二次方程 (quadratic equation) :
12x² - 16kx + 5k² = 0
得 x = 5k/6 or x = k/2

方程式的兩根 (two roots)是5k/6 和k/2,但題目已知其中一根 (one root) 等於 3/2。
即5k/6 = 3/2 或k/2 = 3/2

1) 若root 5k/6 = 3/2,則
5k/6 = 3/2
k = 9/5 (rejected) ...... 因 k 為整數 (integer)

2) 若root k/2 = 3/2,則
k/2 = 3/2
k = 3


我的解法:
3/2 is a roots of the equation.
Hence, put x = 3/2 into the equation :
12x² - 16kx + 5k² = 0
12(3/2)² - 16k(3/2) + 5k² = 0
5k² - 24k + 27 = 0
(k - 3)(5k - 9) = 0
k = 3 or k = 9/5 (rejected forkshould be an integer)
參考: 賣女孩的火柴
2012-09-03 5:22 am
a)12x^2-16kx+5k^2=0 is a quadratic equation in x where k is an interger.Itis
given that 3/2 is a root of the equation.By using the quadratic formula,find
a)the value of k
Sol
12x^2-16kx+5k^2=0
x={16k+/-√[(16k)^2-4*12*5k^2]}/24
=[16k+/-4k]/24
x=5k/6 or x=k/2
(1) 5k/6=3/2
k=(3/2)*(6/5)=18/10=9/5 (reject) k is an interger
(2) k/2=3/2
k=3
b)the other root of the equation
Sol
12x^2-48x+45=0
4x^2-16x+15=0
(2x-5)(2x-3)=0
x=5/2 or x=3/2
the other root of the equation=5/2




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