Question

If the 4th
term of A.p is - 15 and 9th term is -30 find the 23th A.P
​

Answer 1

GIVEN :

4th term of A.p is = - 15

9th term = -30

TO FIND :

the 23th term of the A.P

FORMULA USED :

Tn ( nth term) = a+ (n-1)d

SOLUTION :

Tn ( nth term) = a+ (n-1)d

4th term = a+ (4-1)d = a+ 3d

-15 = a+ 3d

a= -15-3d ........ (1)

9th term = a+ (9-1)d = a+ 8d

-30 = a+ 8d

-30-8d = a.........(2)

equating (1) and (2) ,

-15-3d. = -30-8d

-15+30= 3d -8d

15= -5d

-3= d

a= -15-3d

a= -15 + 9

a= -6

23rd term = a+(23-1)d

= -6+ (22×-3)

= -6 + (-66)

= - 72

ANSWER : -72