Question

in an ap first term is 12 nth term is -1 and sum of 1st nth terms is 66 find the number of terms and common difference

Answer 1

a = 12,

An = - 1

Sn = 66

An = a + ( n - 1 ) d

-1 = 12 + ( n - 1 ) d

( n - 1 ) d = - 13

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

66 = n/2 [ 2( 12 ) + ( - 13 )]

66 = n/2 [ 24 - 13 ]

66 = n/2 [ 11]

n = 66 × 2 / 11

n = 6 × 2 = 12

Number of terms, n = 12

( n - 1 ) d = - 13

( 12 - 1 ) d = - 13

11d = - 13

d = - 13 /11

Common difference, d = - 13/11

Answer 2

Answer:

Given that:

a12=-1

S=66

To find :

n=? d=?

Solution=

a12=a+11d

-1=a+11d.........eq 1

S=n/2*2a+(n-1)d

66=1/2(2a)

66*2=2a

a=66

Putting value of a in eq 1

a+11d=-1

66+11d-1

11d=1-66

11d=-67

d=-67/11

About nth term i didn't get it????