Question

Second and fourth term of an A. P is 12 and 20 respectively. Find the sum of first 25 terms of that A. P

Answer 1

Answer:

A2=a+d=12

A4=a+3d=20

=. -2d=-8

d=-8/-2=4

a+4=12=12-4=8

S25=25/2{2(8)+(25-1)4}

=25/2{16+24*4}

=25/2{16+96}

=25/2{112}

=25*56

=1400

hope this answer useful to you

Answer 2

2nd a=12

4th a=20

2nd a= a+d

12=a+d

=a+d=12 take it as a equation 1

4th=a+3d

20=a+3d

=a+3d=20 take it as a equation 2

sub. eqn1 and eqn2

a+d=12

a+3d=20

-- -- --

=2d= -8

=d=-4

put in eqn 1