How many terms of the AP 2, 5, 8, 11, ... must be added to obtain a sum of 155?

Question

G1 Physics · Accepted Answer

Sₙ=n/2[4+3(n−1)]=n(3n+1)/2. Set equal to 155: 3n²+n−310=0=(n−10)(3n+31). Positive n=10.