First of all, I am new ,so hello.
now to the topic:
is the formula for direct variation y=kx
and could you please post examples?
This is correct. y varies directly as x.
k is called the constant of proportionality or the constant of variation.
Google: direct variation - Google Search
Example: d = rt (distance = rate X time)
Let r be the constant of variation.
The distance (d) varies directly as time (t) if r remains constant. In other words, when t increases, d increases. When t decreases, d decreases.
Follow some of the links I gave you and you'll see other examples.
Also, investigate inverse and joint variation as well.