For the first make you use the cosine rule
can you finish that off ?
for the second part use the sine rule
note
The diagram shows triangle ABC in which AC=4cm, BC=5cm and ACB=60 degrees.
a) Show that AB = Square root of 21
b) find the value of sin (angle ABC) in the form k(square root)7 where k is an exact fraction
Anyone know how to do this?