幫我計幾題數 ...急...

(1) 14x2 +15 -31x=0
14x2 – 31x + 15 = 0
x = [31±√[(-31)2-4(14)(15)]] / 2(14)
= (31±11) / 28
x = 3 / 2 或 5 / 7
(2) 35x2 - 9x - 2=0
x = [9±√[(-9)2-4(35)(-2)]] / 2(35)
= (9±19) / 70
= 2 / 5或 –1 / 7
(3) x2-7x+11=0
x = [7±√[(-7)2-4(1)(11)] / 2(1)
= (7±√5) / 2
(4) 2x2+5x-6=0
x = [-5±√[(5)2-4(2)(-6)] / 2(2)
= (-5±√73) / 4

2007-10-03 23:12:12 補充：
下面的第3,4題漏了括號: 是(7±√5) / 2和(-5±√73) / 4 而不是7±√5/ 2和-5±√73 / 4 。

2007-10-03 23:58:02 補充：
二次公式: x = [ - b±√(b^2 - 4ac )] / 2a

(1)
14x^2 + 15 - 31x = 0
14^2 - 31x + 15
(2x - 3)(7x - 5) = 0
(2x - 3) = 0 or (7x - 5) = 0
x = 3/2 or x = 5/7

(2)
35x^2 - 9x - 2 = 0
(5x - 2)(7x + 1) = 0
(5x - 2) = 0 or (7x+1) = 0
x = 2/5 or x = -1/7

(3)
X^2 - 7x + 11 = 0
x = -(-7)±√(-7)^2 - 4 (1) (11) / 2(1)
x = 7±√5 / 2
x = 7+√5 / 2 or x = 7-√5 / 2

(4)
2x^2+5x-6=0
x = - (5)±√(5)^2 - 4(2)(-6) / 2(2)
x = -5±√73 / 4
x = -5+√73 / 4 or x = -5-√73 / 4