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Properties of Triangles

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=225 cosA=522 (2)

In ABC, by cosine rule, cosA=b2+c2a22bc

i.e. 522=b2+c24b292bc [by equations (1) and (2)]

i.e. 52bc=59b2+c2

i.e. c2(52b)c+59b2=0

i.e. c=52b±5b2220b292

i.e. c=52b±532b2

i.e. c=522b±562b

i.e. c=4562b or c=2562b

i.e. c=2(532b) or c=532b

There are two values to the third side c, one of which is double the other.

Hence proved.