Question

sum of 1st 10 terms in A.P is 17 and sum of next 10terms is 475 find a and d

Answer 1

s10 =17 and s(20-10)= 475
s20=475+s10= 475+17=492
s20= 20(2*a+19d)/2
s20=10(2a+19d)
492=20a+190d------------------(1)
s10=10(2a+9d)/2
17= 20a+90d/2
34=20a+90d----------------------(2)
subtract (2) from (1)
20a+190d=492
20a+90d=034
(-). (-). (-)
0 + 100d=458
d=458/100
d=4.58
substitute value of d in 1
492=20a+190*4.6
20a= 492-874
20a=-382
a=-382/20
a=-19.1