Question

the common diiference of an ap is 4 . the last term is 72 and the sum of all terms is 600. find the first term and the number of terms​

Answer 1

a = - 24 ; n =25

a = 28 ; n = 25

Explanation:

n = l - a/d + 1

d = common difference = 4

Last term = l = 72

n = 72 - a/4 + 1

4n = 72 - a + 4

4n = 76 - a

a = 76 - 4n - - - (1)

Sum of terms :

Sn = n/2(a + l)

Sn = 600

600 = n/2(a + 72)

1200 = n(76-4n +72)

1200 = 148n - 4n

4n² - 148n + 1200

Solving the quadratic equation :

Roots of n are :

12 and 25

From (1)

n = 12

a = 76 - 4(12) = 28

n = 25

a = 76 - 4(25) = - 24

Hence,

a = - 24 ; n =25

a = 28 ; n = 25