(1).∵cosA=-4/5,∴A是钝角,故sinA=√(1-16/25)=3/5
由正弦定理得AC/sinB=BC/sinA,即sinB=AC*sinA/BC
=2*(3/5)/3=2/5.
cosB=√(1-4/25)=√21/5
(2).sin(2B+∏/6)=sin2Bcos(∏/6)+cos2Bsin(∏/6)
=(√3/2)sin2B+(1/2)cos2B
=(√3)sinBcosB+(1/2)(2cos^2B-1)
=(√3)sinBcosB+cos^2B-1/2
=(√3)(2/5)(√21/5)+(√21/5)^2-1/2
=(17+4√63)/50
祝你学习愉快