在△ABC中,角A,B,C所对的边为a,b,c,满足:sin(A-B)+2cosAsinB=-2sin2C,且16a2+16b2-13c2=0.若

2024-11-25 18:43:49
推荐回答(1个)
回答1:

由sin(A-B)+2cosAsinB=-2sin2C,
∵sinAcosB-cosAsinB+2cosAsinB=sin(A+B)=sinC,
∴sinC=-2sin2C=-4sinCcosC,
∵sinC≠0,∴cosC=-

1
4
,∴sinC=
1?cos2C
=
15
4

∵16a2+16b2-13c2=0.∴?
1
4
=cosC=
a2+b2?c2
2ab
=
a2+b2?
16
13
(a2+b2)
2ab
,化为6(a2+b2)=13ab.
∵△ABC的面积为
3
15
4
,∴
1
2
absinC
=
1
2
ab×
15
4
=
3
15
4
,化为ab=6.
∴(a+b)2=a2+b2+2ab=
13
6
ab+2ab
=
25
6
×6
=25.
∴a+b=5.
故选:A.