Typesetting math: 100%

Wednesday, December 19, 2018

Calculus: Maximum and minimum

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=24cosθ7sinθ

For stationary value , dydθ=0

            24cosθ7sinθ=0

                        24cosθ=7sinθ

                            tanθ=247

     d2ydθ2=24sinθ7cosθ

            =cosθ (24tanθ7)

            =cosθ (57677)

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

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