The Dubois formula relates a person's surface area, s, in meters2, to weight, w, in kg, and height, h, in cm, by (a) What is the surface area of a person who weighs 60 kg and is 160 cm tall? (b) The person in part a stays constant height but increases in weight by 0.4 kg/year. At what rate is his surface area increasing when his weight is 64 kg?

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Answer

given,  

Dubois formula  

    [tex]S = 0.01 w^{0.25}h^{0.75}[/tex]  

a) mass = 60 Kg  

   height = 1.6 m

     [tex]S = 0.01 \times 60^{0.25}\times 160^{0.75}[/tex]  

          S = 1.252 m²  

   hence, surface area of a person is equal to 1.193 m² .  

b) [tex]\dfrac{dw}{dt} = 0.4\ kg/year[/tex]  

   weight = 60 Kg  

now,  

    [tex]\dfrac{dS}{dt} = 0.01(0.25)(\dfrac{h}{w})^{0.75}\dfrac{dw}{dt}[/tex]

    [tex]\dfrac{dS}{dt} = 0.01(0.25)(\dfrac{160}{60})^{0.75}\times 0.4[/tex]

    [tex]\dfrac{dS}{dt} = 0.00209\ m^2/year[/tex]  

The surface area of the person using Dubois formula is 1.252 m².

The rate of change in the surface area of the person is 1.988 x 10⁻³ m²/year.

Surface area using Dubois formula

The surface area of the person can be determined using Dubois formula as shown below;

[tex]BSA = wt(kg)^{0.25} \times ht(cm)^{0.75} \times 0.01\\\\BSA = (60)^{0.25} \times 160^{0.75} \times 0.01\\\\BSA = 1.252 \ m^2[/tex]

Rate of change in surface area

The rate of change in the surface area of the person is calculated as follows;

[tex]\frac{d(BSA)}{dt} = 0.01 \times 0.25 \times (\frac{h}{w} )^{0.75} \times \frac{dw}{dt} \\\\\frac{d(BSA)}{dt} = 0.01 \times 0.25 \times (\frac{160}{64})^{0.75} \times 0.4\\\\\frac{d(BSA)}{dt} = 1.988 \times 10^{-3} \ m^2/year[/tex]

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