Question

I need help putting up the equation


A brother and sister have weight (pounds) that varies as the cube of height (feet) and they share the same proportionality constant. The sister is 6 feet tall and weighs 170 pounds. Her brother is 6'4" tall. How much does he wait?

Answer 1

weight varies as the cube of height

so basically
weight = height^3

Answer 2

and a proportionality constant :)

Answer 3

yes, what about the term , proportionality constant ?

Answer 4

weight = constant * height^3

Answer 5

Make units to how much an inch weights. Then compare those units to the boy.

Answer 6

@HyperChemist 12 Inch = 1 foot ?

Answer 7

Yes

Answer 8

Sorry phanta, you must of misunderstood something, which is weird because it's really clear. Divide the height of the person to the weight. Hence to make 1 inch equal whatever weight you get. Then, multiply how much one inch in height weights to the height of the boy.

Answer 9

ok,so the sister is
6 x 12 = 60+12 = 72 Inches

and the brother is
6 ft + 4 inch = 72 + 4 = 76 inches

Answer 10

That is indeed one way to solve the problem ,

but I'm looking for the solution where we use the equation , weight = k ( height)^3

then getting the weight of the boy.

Answer 11

wait I'll re-write with k instand of "constant"

Answer 12

@phanta_seea , what is the equation for the boy's weight ?

Answer 13

:)

Answer 14

we want to get the constant k to determine the boy's weight:

weight = k * height^3
weight / k = height^3
k/ weight = 1/ height^3 <- reciprocal of the above
k = weight/height^3

Answer 15

@yamyam70 the equation for the boy's weight is just this:

weight = k ( height)^3

however we need to know the proportionality constant to be able to gete the weight

Answer 16

weight =the value of k from girl times (6'4")^3 ?

Answer 17

yes :)

Answer 18

@phanta_seea much thanks man :)

Answer 19

original equation:
weight = k * height^3

solve for k, by dividing height on both sides
k = weight/height^3

plug in girl values:
k = 170 pounds /(72 Inch)^3 with 6 ft = 72 Inch

Answer 20

@phanta_seea thanks :))

Answer 21

no problem

Answer 22

something is wrong man

Answer 23

oh nevermind