Results 1 to 4 of 4

Math Help - Couple of problems. help please

  1. #1
    Member
    Joined
    Feb 2009
    Posts
    81

    Couple of problems. help please

    hi.
    can someone please help with the following:

    1. The sides of a cuboid measure 1 cm, a cm and b cm. Its volume is 12cm^3 and its surface area is 38 cm^2.
    Find a and b

    2. Show that y=x+c is a tangent to x^2 + y^2=4 if, and only if, c^2=8. (Show that if y=x+c is a tangent then c^2=8, and also that if c^2 = 8 then y= x+c is a tangent)

    thanks

    with the second one ive got as far as
    i know the discriminant must equal 0 and the quadratic is 2x^2 +2cx +(c^2-4)=0,
    and from the first statment 4c^2-8(c^2-4)=0
    Last edited by sammy28; April 30th 2009 at 01:15 AM.
    Follow Math Help Forum on Facebook and Google+

  2. #2
    Master Of Puppets
    pickslides's Avatar
    Joined
    Sep 2008
    From
    Melbourne
    Posts
    5,236
    Thanks
    28
    Quote Originally Posted by sammy28 View Post
    hi.
    can someone please help with the following:

    1. The sides of a cuboid measure 1 cm, a cm and b cm. Its volume is 12cm^3 and its surface area is 38 cm^2.
    Find a and b

    Consider dimensions of the cubiod to be length(L), width(W) and height(H).

    lets make (from your information) L = 1, W = a, H = b.

    Now solve these equations

    1:
    V = L \times W \times H

    12 = 1 \times a \times b

    a = \frac{12}{b}

    2:

    SA = 2(L \times W+W \times H+H \times L)

    38 = 2(1 \times a+a \times b+b \times 1)

    38 = 2a+2ab+2b

    Now sub the result in 1 into the result in 2
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Member
    Joined
    Feb 2009
    Posts
    81
    thanks pickslides, for some reason i had drawn a net and written the area as:

    ab=12
    2ab+4b=38

    then tried elimination to solve and got wrong answer

    if i solve the quadratic in your answer, ie b^2-7b+12=0 then all is well

    i need to compare the net with your forumula.

    thanks
    Follow Math Help Forum on Facebook and Google+

  4. #4
    Master Of Puppets
    pickslides's Avatar
    Joined
    Sep 2008
    From
    Melbourne
    Posts
    5,236
    Thanks
    28
    Quote Originally Posted by sammy28 View Post
    thanks pickslides, for some reason i had drawn a net and written the area as:

    ab=12
    2ab+4b=38

    then tried elimination to solve and got wrong answer

    if i solve the quadratic in your answer, ie b^2-7b+12=0 then all is well

    i need to compare the net with your forumula.

    thanks

    b^2-7b+12=0


    agreed...
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. [SOLVED] Another Couple of Problems
    Posted in the LaTeX Help Forum
    Replies: 5
    Last Post: October 11th 2010, 09:26 AM
  2. Help on a couple of problems
    Posted in the Differential Equations Forum
    Replies: 1
    Last Post: March 25th 2010, 01:36 AM
  3. couple problems i have
    Posted in the Calculus Forum
    Replies: 2
    Last Post: April 20th 2009, 07:21 PM
  4. Couple more problems
    Posted in the Trigonometry Forum
    Replies: 1
    Last Post: January 16th 2007, 06:09 PM

Search Tags


/mathhelpforum @mathhelpforum