Joint direct variation

Optional extension

The quantity \(A\) varies directly with \(l\) and \(h\), so \(A=klh\). Given \(A=90\) when \(l=5,h=3\):

1. express \(A\) in terms of \(l,h\)[2]
2. find \(l\) when \(A=504,h=7\).[2]