Question

find the sum of first 25 terms of and ap whose nth term is 1-4n

Answer 1

Sum=n/2(a+l)


first term(a). = 1-4×1=-3
nth term(n=25)(l). =1-4×25=-99

a=-3
l=-99

sum= 25/2[-3+(-99)]
= 25/2×(-102)
= 25×(-51)
= -1275

Answer 2

Answer:

S₂₅=-1275

Step-by-step explanation:

lets take n=1

∴1-4(1)

∴1-4

∴-3

lets take n=2

∴1-4(2)

∴1-8

∴-7

lets take n=3

∴1-4(3)

∴1-12

∴-11

AP is -3,-7,-11,...

∴a=-3

∴d=-7-(-3)

∴d=-4

∴S₂₅=n/2{2a+(n-1)d}

∴      =-25/2{2(-3)+(25-1)(-4)}

∴      =-25/2{-6+24(-4)}

∴      =-25/2{-6-96}

∴      =-25/2×-102

∴      =-25×51

∴ S₂₅=-1275