[محمود طه القالع]

In an arithmetic sequence the sum of its first and third terms = 10
and its tenth term = 21, how many terms starting from the first
should be taken to make the sum = 440 ?



when s1 + s3 = 10
then
a + a+ 2r = 10
then
2a + 2 r = 10
then
a +r = 5
and
the tenth term = 21
then
a + 9 r = 21
subtract the two equ
then
8r = 16
then
r = 2
and
a = 3
thearithmetic sequence
is
( 3 ,5, 7 , ....................... )