The diagram below show three right-angle triangles, where BC=14,GF=10,DE=7 and ∠BCA=∠BDE=∠FGD=θ . Find the maximum possible value of AB+BD+DF.
Solution
BC=14,GF=10,DE=7
∠BCA=∠BDE=∠FGD=θ
Let y=AB+BD+DF
=14sinθ+7cosθ+10sinθ
=24sinθ+7cosθ
dydθ=24cosθ−7sinθ
For stationary value , dydθ=0
24cosθ−7sinθ=0
24cosθ=7sinθ
tanθ=247
d2ydθ2=−24sinθ−7cosθ
=−cosθ (24tanθ−7)
=−cosθ (5767−7)
Since θ is acute , d2ydθ2<0 (maximum)
The maximum possible value of AB+BD+DF
=24sinθ+7cosθ
=24(2425)+7(725)
=57625+4925
=62525=25
No comments:
Post a Comment