F.4 maths唔識><HELP please

1.
x=4y+9 ......(1)
x=4-y² ......(2)

equates (1) and (2)

4y+9 = 4-y²
y² + 4y + 5 = 0
using : [-b±√(b²-4ac)]/2a

y = [-4±√(16-20)]/2
y = (-2+4i) or (-2-4i)


2014-03-02 16:20:01 補充：
using : [-b±√(b²-4ac)]/2a

y = [-4±√(16-20)]/2
y = [-4±√(-4)]/2
y = [-4±(√4)(-1)]/2
y = [-4±(2)√(-1)]/2
y = (-2+i) or (-2-i)

2014-03-02 16:28:12 補充：
Let the number be XY

(10X+Y) - XY = 18 ...... (1)　　　　　[ e.g. 36 = 3X10 + 6 ]
X+Y = 13
X = 13-Y ......... (2)

Sub (2) into (1)

10(13-Y) + Y + (13-Y)Y = 18
130 - 10Y + Y + 13Y - Y² = 18
130 + 4Y - Y² = 18
Y² - 4Y - 112 = 0

2014-03-02 16:40:36 補充：
更正 :

Let the number be XY

(10X+Y) - XY = 18 ...... (1)　　　　　[ e.g. 36 = 3X10 + 6 ]
X+Y = 13
X = 13-Y ......... (2)

Sub (2) into (1)

10(13-Y) + Y - (13-Y)Y = 18
130 - 10Y + Y - 13Y + Y² = 18
130 - 22Y + Y² = 18
Y² - 22Y + 112 = 0
(Y-14)(Y-8)=0
Y=14 or Y=8

2014-03-02 16:42:28 補充：
When Y = 14 ; X+Y=13; X=-1 (reject)

When Y = 8; X+Y=13; X=5

The integer is 58

checking : 58 - 5X8 = 58 - 40 = 18

2014-03-02 16:50:30 補充：
3.

Let the present age of Mr.Wong and his daughter be W and D

W - D = 30
W = 30 + D .... (1)
(W-2) = (D-2)²
W-2=D²-4D+4 .... (2)

Sub (1) into (2)

(30+D) - 2 = D²-4D+4
D + 28 = D²-4D+4
D² - 5D -24 = 0
(D-8)(D+3) = 0
D = 8 or D = -3 (reject)

when D = 8 ; W = 30 + D ; W = 38

2014-03-02 16:51:53 補充：
Checking :

W - D = 38 - 8 = 30

2 years ago : W-2=36 ; D-2=6 ; 36 = 6²