唔識計呀..

f (1) = - 3
a (1+2) (1-2) +b = - 3
-3a + b = -3-----------------------(1)

f(3) = 13
a (3+2) (3-2) +b = 13
5a +b = 13------------------------(2)

(2)-(1): 8a = 16
a=2
sub a=2 into (2):
10+b=13
b=3

When f(1) = -3

a(1+2)(1-2)+b = -3
-3a + b = -3
b = -3 + 3a -- (1)

When f(3) = 13

a(3+2)(3-2)+b=13
5a + b = 13 -- (2)

Substitute (1) into (2)

5a -3 +3a = 13
8a = 16
a = 2

Put a = 2 into (1)

b = -3 + 3(2)
b = 3
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Proof by using f(1) = -3

2(1+2)(1-2) + 3
= -3

因f(x)=a(x+2)(x-2)+b
代入ｆ（１）＝-3
-3=a(1+2)(1-2)+b
-3=-3a+b
b=3a-3

代入f(3)=13
13=a(3+2)(3-2)+b
13=5a+b
b=13-5a

因b=3a-3 & b=13-5a
所以3a-3=13-5a
3a+5a=13+3
8a=16
a=2

a=2代入b=3a-3
b=6-3
b=3

f (1) =a(1+2)(1-2)+b=-3
-3a+b=-3
b=-3+3a....(1)
f(3) =a(3+2)(3-2)+b=13
5a+b=13....(2)
將(1)代入(2)
5a+(-3+3a)=13
8a=16
a=2...(3)
將(3)代入(1)
b=-3+3x2
b=3

a=2 b=3

f (1) =a(1+2)(1-2)+b=-3
-3a+b=-3
b=-3+3a....(1)
f(3) =a(3+2)(3-2)+b=13
5a+b=13....(2)
將(1)代入(2)
5a+(-3+3a)=13
8a=16
a=2...(3)
將(3)代入(1)
b=-3+3x2
b=3

a=2 b=3