Anonymous
Anonymous asked in Science & MathematicsMathematics · 1 decade ago

Maths Problem- Need Urgent Help!!?

Hey there, i need this maths problem finished and pronto!

It is regarding proportion.

IMPORTANT- Answers should include full method working for each part of the question- clearly labelled and correct.

Here comes the question:

y is directly proportional to the square of x.

y=72 when x=2

a) Express y in terms of x

b) Work out the value of y when x=1/2.

Please remember full working all clearly labelled.

Many thanks- and be speedy!

7 Answers

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  • Anonymous
    1 decade ago
    Best Answer

    First of all, if y is directly proportional to the square of x, you can write this as

    y = kx^2, where k is a constant of proportionality.

    Because you have been given two values, it is possible to work out the value of k.

    (i've used * for multiplication to avoid confusion with x!)

    72 = k * 2^2

    72/4 = k

    k = 18

    So your answer to a is y = 18x^2

    For b simply plug in 1/2 to your equation:

    y = 18 * (1/2 ^2)

    y = 18 * 1/4

    y = 4.5

  • 4 years ago

    I can't answer that, because for that fraction, I don't know if the numerator is 5, (4+5), (2*4+5). Same with the denominator. However, if I assume that you mean: 2 x 4 + (5 / 2) - 3 x 4 + 3 = then follow the order of operations, PEMDAS. That stands for Parentheses, Exponents, Multiplication, Division, Addition Subtraction. Multiplication and division are done at the same time but from left to right, as are addition and subtraction. So first, you can't do anything with what is in parentheses, and there are no exponents, so start with multiplication and division: (2*4) + (5/2) -(3*4) +3 = 8 + (5/2) -12 +3 Now, add or subtract from left to right: (8 5/2) -9 convert to improper fractions with a denominator of 2: 21/2 - 18/2=3/2 So the answer is 3/2.

  • 1 decade ago

    Since y is proportional to x^2, call that proportion "k". Then:

    y = k*x^2

    If y = 72 when x = 2, plug those in to find k:

    72 = k*2^2

    72 = 4*k

    18 = k

    So the answer to part a) is y = 18*x^2

    For b), just plug in 1/2 for x and solve:

    y = 18*(1/2)^2

    y = 18*(1/4)

    y = 18/4 = 9/2 = 4.5

  • Anonymous
    1 decade ago

    To find out the proportion solve the equation

    2-squared * N = 72

    4N = 72

    N = 18

    so

    Y = X-Squared * 18

    When x - 1/2,

    (1/2)-squared * 18 = y

    1/4 * 18 = y

    y = 4.5

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  • mom
    Lv 7
    1 decade ago

    y is directly proportional to the square of x.

    y=72 when x=2

    y=kx²

    72=k(4)

    18=k

    --------------

    a) Express y in terms of x

    y=18x²

    -----------------

    b) Work out the value of y when x=1/2.

    y=18(1/4)

    y=4.5

  • 1 decade ago

    a)

    y = 18x^2

    b)

    y(1/2) = 18 * 1/4 = 18/4 = = 9/2 = 4 1/2 = 4.5

  • Anonymous
    1 decade ago

    Do your own questions!

    ask sombody in person or tell the teacher you dont understand

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