if a=3i j 4k and b=i-j k, then find a.b

If sinx= -1/4. Where x lies in III quadrant. Find sinx/2,
cosx/2, tanx/2.
In an infinite geometric progression, the sum of first two terms is 6 and the every term is 4 times the sum of all terms that follow it. Fine the Geometric progression and sum to infinity.