If 2b=3a and tan2A=35, prove that there are two values to the third side, one of which is double the other.
Given: 2b=3a ⟹ a=2b3 ⋯(1)
and tan2A=35
i.e. 1+tan2A=sec2A=1+35=85
i.e. secA=2√2√5 ⟹ cosA=√52√2 ⋯(2)
In △ABC, by cosine rule, cosA=b2+c2−a22bc
i.e. √52√2=b2+c2−4b292bc [by equations (1) and (2)]
i.e. √5√2bc=59b2+c2
i.e. c2−(√5√2b)c+59b2=0
i.e. c=√5√2b±√5b22−20b292
i.e. c=√5√2b±√53√2b2
i.e. c=√52√2b±√56√2b
i.e. c=4√56√2b or c=2√56√2b
i.e. c=2(√53√2b) or c=√53√2b
∴ There are two values to the third side c, one of which is double the other.
Hence proved.