contestada

If y varies inversely as x^2 and y = 3 when x = 18, what is y when x = 3?

a.

6


b.

108


c.

54


d.

27

Respuesta :

y*x^2=k
y=3, x=18 ⇒ 3*18^2=3*324=972=k
x=3, 972=y*x^2=y*9
y=972/9
y=108

The value of y when x= 3 is 108.

Coefficient of proportionality:

Two sequences of numbers, often experimental data, are proportional or directly proportional if their corresponding elements have a constant ratio, which is called coefficient of proportionality.

Inversely proportional:

When the value of one quantity increases with respect to decrease in other or vice-versa, then they are said to be inversely proportional.

According to the given question

y varies inversely as [tex]x^{2}[/tex].

⇒  [tex]y = \frac{k}{x^{2} }...(i)[/tex]

where, k is the coefficient of proportionality.

Given, y = 3 when x = 18

⇒ [tex]3 = \frac{k}{324}[/tex]

k = 3 × 324 =972

Therefore, for the value of y when x = 3

⇒ y = [tex]\frac{972}{(3)^{2} }[/tex]           (substitute x =3 and k = 972 in equation (i))

⇒ y = [tex]\frac{972}{9} = 108[/tex]

Hence, the value of y when x = 3 for the expression [tex]y=\frac{972}{x^{2} }[/tex] is 108.

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