Question

If an=3-4n,show that a1,a2,a3...........form an A.P. also,find s20

Answer 1

Given
an =3-4n
a1=3-4(1)=-1
a2=3-4(2)=-5
a3=3-4(3)=-9
d=-4
a20= a+19d =-1+19(-4)
= -1-76
=-77
hope this help
s20= n/2(2a +(n-1)d)
=20/2(-2+19(-4)
=10×(-2-76)
=10×-78=-780
Check if this so correct?

Answer 2

Given,
an=3-4n
Proof÷a1=3-4×1=3-4= -1
a2=3-4×2=3-8= -5
a3=3-4×3=3-12= -9
a4=3-4×4=3-16= -13
so to prove that they are in A.P.the common difference (d)should be same
d=a2-a1= -5-(-1)= -5+1= -4
to conform d you van take a3-a2

Here,
s20=n/2 {2a+(n-1)d}
s20=20/2 {2×-1+(20-1)-4}
s20=10{-2+19×-4}
s20=10{-2-76}
s20=10×-78
s20=-780

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