+251 
Hello.
I've been working at a 2D kinematics problem for a couple of days now, trying to work out how to do it:
you can view it here:
http://physics.stackexchange.com/questions/101508/finding-an-equation-to-solve-for-the-velocity-of-a-swimmer-crossing-a-river
I've worked out the idea of how to get the answer, however I just can't seem to get there.
I was hoping someone could explain how they went from step 1 to step 2. I can see some factoring going on but just can't quite work it out.
Thank you for your help.
+9675 Step 1: d2⋅vs⋅cos45∘=d1⋅(vr−vssin45∘)
They multiplied out the bracket on the right hand side:
d2⋅vs⋅cos45∘=d1⋅vr−d1⋅vssin45∘
and then they add d1 . vs sin45 each side:
d2⋅vs⋅cos45∘+d1⋅vssin45∘=d1⋅vr
Factor out the common factor(vs):
vs⋅(d2⋅cos45∘+d1sin45∘)=d1⋅vr <-- Step 2!!
