Question

find the sum of the following arithmetic progression 40, 60. 80, - - - - -- to 25th terms​

Answer 1

Answer:

$Sn=\frac{n}{2}[2a+(n-1)d] \\ = \frac{25}{2}[2\times40+(25-1)20] \\ =\frac{25}{2}[80+(24)20] \\=\frac{25}{2}[80+480] \\=\frac{25}{2} \times 560 \\ S25 = 7000$