唔該幫幫忙/???

T(n) =an2+bn T(3)=9和T(6)=72
A) T(3) = a3^2+3b = 9
9a = 9-3b
a = 3(3-b)/9
a = (3-b)/3

T(6) = a6^2+6b = 72
36a+6b = 72
36[(3-b)/3]+6b = 72 ------ [a=(3-b)/3]
12(3-b)+6b = 72
36-12b+6b = 72
36-6b = 72
36-72 = 6b
-36 = 6b
b = -6

a = (3-b)/3
a = [3-(-6)]/3
a = 9/3
a = 3

T(n) = 3n^2-6n

B) T(1) = 3(1^2)-6(1) = -3
T(2) = 3(2^2)-6(2) = 0

a(3)^2+3b=9----------1
a(6)^2+6b=72--------2
put 1 into 2,a=3
b= -6

T(1)=3-6=-3
T(2)=4x3-6x2=0