Choosen As Best Answer

根與係數
a + b = -3a ...(1)
ab = 4b ...(2)
從(2)可得 a = 4 代回(1)
4 + b = -12
b = -16

******

x^2 - 5x = -1
a^2 - 5a = -1
a^2 + 1 = 5a

(1)
3a + 3/a - 7
= (3a^2 - 7a + 3)/a
= [3(a^2 + 1) - 7a]/a
= 8a/a = 8
(2)
(x+1)(x+2)(x-6)(x-7)
= (x^2 -5x -6)(x^2 -5x -14)
= (-1-6)(-1-14)
= 7*15 = 105

訪客

第一題為什麼是a+b=-3a?
第二的2是怎麼變成數字的?

Balm of Gilead

根與係數 ax^2 + bx + c = 0 有兩根分別為 α、β,則可得知
α+β = -b/a
αβ = c/a

至於第二題
因為題目已知 x^2 - 5x = -1
替換進所求的多項式

訪客

謝啦

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