Please do the last part "Hence state with a brief reason whether angle A+B is acute or obtuse"
sin(A+B) = 8/15 +[sqrt(5) /5] ------------positive sine value
So, (A+B) is in the 1st or 2nd quadrants.
cos(A+B) = 4sqrt(5) /15 -[2/5]
= 4sqr(5) /15 -[6/15] --------------------positive cosine value
So, (A+B) is in 1st or 4th quadrants.
Hence, (A+B) is definitely in the 1st quadrant.
Therefore, (A+B) is an acute angle.