how many terms of an a.p. -10,-7,-4....... must be added to get the sum 104?

S_n =n/2 (2a + (n-1)d )

104 =n/2(-20+3n-3)

208 = -23n + 3n²

3n² -23n-208 =0

apply quadratic equation

n=13 or -16/3

n cant be -ve

therefore n =13