expressing sin(x) in terms of tan(x)

I saw this in a list of previous examination questions today and became intrigued. It asks:

"The acute angle x radians is such that tan(x) = k. Express, in terms of k, sin(x)."

The obvious answer is k cos(x). But as a 2 mark question I get the feeling that something else is required (and I'm not sure if cos(x) counts as 'in terms of x').

I have feeling there's something deceptively obvious here than I'm missing. Can anyone give me hints, or am I overthinking this?