Question

The common difference of an AP is 4 and its last term is 72. If the sum of all the terms id 600 .find the first term and the numbenumber of terms

Answer 1

Answer:

a=28 and n=12

a= -24 and n= 25

Step-by-step explanation:

n = l-a/d + 1

n= 72-a/4 + 1

4n = 72-a+4

4n = 76-a

a= 76-4n (1st equation)

Sn = n/2(a+l)

600 = n/2 (a+72)

1200 = n(a+72)

1200 = n(76-4n+72)

1200 = 148n - 4n²

4n²-148n+1200=0

divide by 4

n²-37n+300= 0

(n-12)(n-25) = 0

n=12,25

a= 76-48 or 76-100

a=28,-24