In the sequence a=1,3,6,10,16 what is the value of a2

Which equation represents a proportional relationship that has a constant of proportionality equal to 1/5

Which graph corresponds to the function f(x) = x2 + 4x – 1?

Offering 30 if a plus b plus c equals 68 and ab plus bc plus ca equals 1121, where a, b, and c are all prime numbers, find the value of abc. the answer is 1978 but i need an explanation on how to get that.