Answer:
a = - 24 ; n =25
a = 28 ; n = 25
Step-by-step 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
