Question

first term and common difference of an a.p. are 10 and -4 respectively find Sn and S25​

Answer 1

Step-by-step explanation:

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

a = 10 d = -4

Sn = n/2 [2(10)+(n-1)(-4)]

= n/2 [20-4n+4]

= n(12-2n) = 12n - 2n^2

S25 = 12(25) - 2(25)^2

= 300 - 2(625)

= 300-1250 = -950