Question

The first term of an A.P. is - 5 and the 6th term is 45. Find Sn.​

Answer 1

Step-by-step explanation:

120

Step-by-step explanation:

Given that :

a = -5, T6 = 45

As we know that :

Tn = a + (n – 1)d

=> T6 = -5 + (6 – 1)d

=> 45 = -5 + 5d

=> 5d = 45 + 5

=> 5d = 50

=> d = 10

Also we know that :

Sn = n/2[ 2a + (n – 1)d ]

=> S6 = 6/2[ 2×(-5) + (6-1)10 ]

=> S6 = 3[ -10 + 50 ]

=> S6 = 3 × 40 = 120

Answer 2

Answer:

n(n-2)

Step-by-step explanation:

ATQ,

a6 = 45

=> a + (6-1)d = 45

=> -5 + 5d = 45

=> 5d = 45+5

=> d = 50/5 = 10.

Now, Sn = n/2 [2a + (n-1)d]

We don't know the value of 'n' for which we need to find the value as it is not given in the question but putting the values of everything we know -

Sn = n/2 [-10 + (n-1)10]

=> Sn = n ( n-1-1)

=> Sn = n (n-2)