Answers:

(i) 

n = 25

First term, a = a₁ = 3

Common difference, d = a₂ – a₁ = 9/2 – 3 = (9 – 6)/2 = 3/2

By using the formula,

S = n/2 (2a + (n – 1) d)

Substitute the values of ‘a’ and ‘d’, we get

S = 25/2 (2(3) + (25-1) (3/2))

S = 25/2 (6 + (24) (3/2))

S = 25/2 (6 + 36)

S = 25/2 (42)

S = 25 (21)

S = 525

∴ The sum of the given AP is 525.

(ii) 

n = 12

First term, a = a₁ = 41

Common difference, d = a₂ – a₁ = 36 – 41 = -5

By using the formula,

S = n/2 (2a + (n – 1) d)

Substitute the values of ‘a’ and ‘d’, we get

S = 12/2 (2(41) + (12-1) (-5))

S = 6 (82 + (11) (-5))

S = 6 (82 – 55)

S = 6 (27)

S = 162

∴ The sum of the given AP is 162.