Secondary 2 Maths

• BillaMax

  4 Jan 10, 22:47

  Secondary 2 Maths question?

  Two quantities, x^2 and y, are in direct proportion. It is known that y = 60 for a particular value of x. Find the value of y when the value of x is doubled.
  
  I do not know how to solve this question any help will be deeply appreciated. Thank you.
  
• -Wanderer-

  4 Jan 10, 22:57

  y is directly proportional to x^2.

  Therefore, y = k x^2

  When y = 60,

  60 = k x^2                        - (1)

  When x is doubled,

  y = k (2x)^2

  y = 4k x^2                        - (2)

  Substitute your (1) into (2),

  y = 4(60) = 240

  It is just a simple simultaneous equation. Combine both to get your y value. Cheers! 

• wee_ws

  5 Jan 10, 10:20

  Hi,

    Understand the keywords in this type of question, like directly proportional and inversely proportional.

    Next, understand the approach, like having to form equation(s) to find the unknown constant of proportionality.

    Thanks.

  Cheers,
  Wen Shih