0抱歉是sin（a+β）=5分之4答：
0
sin(a+b)=4/5
1)
sin(2b)=cos(π/2-2b)
=cos[2(b-π/4)]
=2cos²(b-π/4) -1
=2*(1/3)²-1
=2/9-1
=-7/9
所以：sin(2b)=-7/9
2）
因為：π/2
因為：π/4
cos(a+π/4)
=cos[ (a+b)-(b-π/4) ]
=cos(a+b)cos(b-π/4)+sin(a+b)sin(b-π/4)
=(-3/5)*(1/3)+(4/5)*(2√2/3)
=-3/15+8√2/15
=(8√2-3)/15
所以：cos(a+π/4)=(8√2-3)/15