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20 14 Anhui college entrance examination science mathematics 16 steps to do the problem
∴a/2sinBcosB=b/sinB？ ∴a=2bcosB=6cosB，

b/sinB=c/sinC？ b/sinB=c/sin( 180 -3B)？ b/sinB=c/sin3B，∴b/sinB=c/sin(2B+B)

∴b/sinb=c/(sin2bcosb+cos2bsinb)∴(2sinbcos？ B+cos2BsinB)/sinB= 1/3

∴2cos？ B+2cos？ B- 1= 1/3？ cosB=√3/3？ ∴a=6cosB=2√3

This is a formula problem,

That's it. Please take it.

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