點先計到呢條數?20分

It is an arithmetic sequence.
T(n) = a + (n-1) d

T(7) = a + 6d = 4 ................... (1)
T(20) = a + 19d = -35 ............. (2)
(2) - (1): 13d = -39, thus d = -3.
Putting into (1): a -18 = 4, thus a = 22.

T(9) = 22 + 8 x (-3) = -2.

T(m) = 22 + (m-1)(-3) = -86
-3(m-1) = -108
m-1 = 36
m = 37

Given that T(7)=4 and T(20)= -35

a) the general term T(n)

問題黎啦...T(n)=a+(n-1)d 個a(first term) 點得番黎? d [T(n+1)-T(n)] 又點得番黎?根本就有2個未知數
用代入法...
T(7)=4
T(n)=a+(n-1)d
T(7)=a+(7-1)d=4
a+6d=4
a=4-6d－１
T(20)= -35
T(20)=a+(20-1)d=-35
a+19d=-35－２
１代入２
4-6d+19d=-35
13d=-39
d=-3
a+6d=4
a+6(-3)=4
a=22


b) the ninth term of the progression
T(9)=22+(9-1)-3=46


c) m if T(m) = -86

唔知佢問咩鬼野...唔通m就係20?
T(m) = -86
T(m)=a+(m-1)d
T(m)=22+(-3)(m-1)
m-1=(-86-22)/(-3)
m-1=36
m=37

4=a+(7-1)d...........1
-35=a+(20-1)d.......2

聯立方程計倒a同d

b)T(9)=a(知)+(9-1)d(知)

c)-86=a(知)+(m-1)d(知)

搞店~

用聯立方程