想問一元二次方程 ---10分(只限今日)

1.x2-34=5x

x^2-34=15x <--係咪15x
x^2-15x-34=0
(x-17)(x+2)=0
(x-17)=0 or (x+2)=0
x=17 or x=-2


2.x2=x+42

x^2=x+42
x^2-x-42=0
(x-7)(x+6)=0
(x-7)=0 or (x+6)=0
x=7 or x=-6

3.4x2+25=0 <--冇real roots 架喎

4x^2-25=0 <--係咪咁
4x^2-5^2=0
(2x-5) (2x+5)=0
(2x-5) =0 or (2x+5)=0
x =5/2 or x=-5/2


4.(x-4)(3x-1)=7x- x2-21

(x-4)(3x-1)=7x- x^2-21
3x^2 -13x +4 =7x- x^2-21
4x^2 -20x +25 =0
(2x -5)^2 =0
(2x -5) =0
x =5/2

5.3(x2-11)/5-30=2/7(x2-60)+6

3(x^2-11)/5-30=2/7(x^2-60)+6
[3(x^2-11)-150]/5=[2(x^2-60)+42]/7
7[3(x^2-11)-150]=5[2(x^2-60)+42]
7[3x^2-33-150]=5[2x^2-120+42]
7[3x^2-183]=5[2x^2-78]
21x^2-1281=10x^2-390
11x^2-891=0
x^2-81=0
(x-9)(x+9)=0
(x-9)=0 or (x+9)=0
x=9 or x=-9


6.(x-7)2=16

(x-7)^2=16
(x-7)^2-16=0
(x-7)^2-4^2=0
[(x-7)-4][(x-7)+4]=0
[x-11][x-3]=0
[x-11]=0 or [x-3]=0
x=11 or x=3

1. x2-34=5x
-34+x2=5x
-34=5x-2x
3x= -34

2. x2=x+42
42=2x-x
x=42

3.4x2+25=0
8x=-25
x=-3.25

4. (x-4)(3x-1)=7x- x2-21
(x-4)(3x-1)=5x-21
3x-1+12x+21=5x
20=5x-3x-12x
20= -10x
5. 3(x2-11)/5-30=2/7(x2-60)+6
1.2x-6.6 -30-6=4/7x-120/7
84/70x-40/70x=6.6+30+6-120/7
22/35x=24.6/7
x=24.6/7*35/22
x=143/22
6. (x-7)2=16
2x-14=16
2x=16+14
x=30/2
x=15