Question

find the sum of 1rst 25 terms. of an AP whose nth term is given by an =2-3n​

Answer 1

Step-by-step explanation:

So, here we can find the sum of the n terms of the given A.P., using the formula,

S n = ( n 2 ) ( a + l )

S 25 = ( 25 2 ) [ ( - 1 ) + 2 - 3 ( 25 ) ]

= ( 25 2 ) [ 1 - 75 ]

= ( 25 2 ) ( - 74 )

= ( 25 ) ( - 37 )

Answer 2

Step-by-step explanation:

an=2-3n

a1=2-3×1=2-3=-1

a2=2-3×2=2-6=-4

d=a2-a1=-4-(-1)=-4+1=-3

a=-1,d=-3,n=25

Sn=n/2{2a+(n-1)d}

S25=25/2{2(-1)+(25-1)(-3)}

=25/2{-2+24(-3)}

=25/2(-2-72)

=25/2(-74)

=25(-37))=

S25=-925