in the diagram (attached), ABCD is a quadrilateral where AD parallel to BC. it is given that AB=9, BC=6, CA=5 and CD=15. show that cos BCA=-1/3 and hence find the value of sin BCA i can do the 'show that' bit but i don't know how to find sinBCA. any help?? also when evaluating log15 + log20 - log12 (all to the base 5) do you do the addition to multiplication conversion first or the subtraction to division first?