Question

if the 21st term of an a.p. 25 find the sum of its first 41 term

Answer 1

Hi Mate!!!

21ist term = a + 20d = 25

Multiple both sides by 2

2a + 40d = 50.... equation 1

Sum of ist 41 terms of an ap is

S41th = 41 / 2 { 2a + ( 40 ) d }

S41th = 41 / 2 { 50 }

S41th = 1025

Have a nice day