问题 填空题
在斜△ABC中,
c
b
+
b
c
=8cosA
,则
tanA
tanB
+
tanA
tanC
=______.
答案

在斜△ABC中,

c
b
+
b
c
=8cosA,故
c2+b2
bc
=8
c2+b2-a2
2bc
,化简可得 3(b2+c2 )=4a2

tanA
tanB
+
tanA
tanC
=
sinAcosB
cosAsinB
+
sinAcosC
cosAsinC
=
sinAcosBsinC+sinBsinAcosC
cosAsinBsinC
=
sinAsin(B+C)
cosAsinBsinC
 

=

sin2A
cosAsinBsinC
=
a2
b2 +c2-a2
2bc
×bc
=
2a2
b2+c2a2
=
2a2
4a2
3
a2
=6,

故答案为:6.

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