Question

if Sn = n^2 - 3n than find the value of T9

Answer 1

S9 = 9²-3*9=81-27=54
S8 =8²-3*8=64-24=40
t9 =s9 -s8=54-40=14

here tn =sn -s(n-1) is the formula I used
AND n, n-1 are subscripts

hopes u can understand.....

Answer 2

Sn = n^2 - 3n

Let n = 1,

S1 = a1 = ( 1 )^2 -3( 1 )

a1 = 1 - 3 = - 2

Let n = 2

S2= a1 + a2 = ( 2 )^2 - 3 ( 2 )

- 2 + a2 = 4 - 6

-2 + a2 = -2

a2 = 0

d = a2 - a1 = 0 - ( - 2 )

d = 2

t9 = a + ( 9 - 1 )d

-2 + 8(2)

-2 + 16

t9 = 14