a∈(0,π/2),
a+π/3∈(π/3,5π/6),
cos(a+π/3)=-4/5
sina
=sin(a+π/3-π/3)
=sin(a+π/3)cosπ/3-cos(a+π/3)sinπ/3
=3/5*1/2+4/5*√3/2
=3/10+2√3/5
sin(a+π/3)=3/5