If m and k are non-zero integers, is m a multiple of k?
(1) (m2+m)/k is an integer.
The catch is what eva said,
Two consecutive integers are co prime !!
if that doesnt strike you, put in some value.
Remember, the difference between an identity and an equation.
An equation is tru for certain values of variable, whereas an identity is true for all values of variables.
Here what we are given is an Identity on the set of integers so put in a value which proves this wrong
And also remember oone more thing, if a value you put into it satisfies doesn't mean that it is true, but if it doesn't satisfiy it does mean that it is not true
Here put m = 3 and take k = 4
m(m+1) = 3*4 = 12, K divides 12 but k doesnt divide 3, hence not sufficient.