Arithmetic series question help please

A farmer has to plant seeds in a triangular field. He knows that the middle row, row 26, needs 4025 seeds, and that the last row needs 7525 seeds. If the number of seeds planted in each row follows and arithmetic series, how many total seeds does he need?

♥♦♣♠
since row 26 is the MIDDLE row,
there would be 25 rows on either of its sides.
so the last row is row ( 26 + 25 ) or row 51.

d = ( 7,525 - 4,025 ) / ( 51 - 26 )
...= 3,500 / 25
...= 140

a_n = a + ( n - 1 ) d

a_26 = a_1 + ( 26 - 1 ) ( 140 ) = 4,025
a_1 + ( 25 ) ( 140 ) = 4,025
a_1 + 3,500 = 4,025
a_1 = 525

S_n = ( n / 2 ) ( a_1 + a_n )

S_51 = ( 51 / 2 ) ( 525 + 7,525 )
..........= ( 25.5 ) ( 8,050 )
..........= 205,275
░░░░░░░░░░░░total seeds needed.

I have already answered this question