m/2 (2a+(m-1)d) = 5m^2+3m
Since I have no idea what n is supposed to be, I'll guess you meant m. So, that means that
a+(m-1)d = 168
m/2 (a+168) = 5m^2+3m
a+168 = 10m+6
a+162 = 10m
So, let's take a=8, making m=17
8+16d=168
d=10
5m^2+3m = 17(5*17+3) = 1496
17/2 (16+16*10) = 1496
So, one sequence is
8, 18, 28, 38, ...
T20 = 8+19*10 = 198
But, you can see that there are other such sequences as well.
The sum of first m terms of an A.P is 5m²+3m. If its nth term is 168. Find the value of m. Also find the 20th term.
1 answer
1 answer