Results 1 to 7 of 7

Math Help - Pythagoras' theorem

  1. #1
    Newbie
    Joined
    Feb 2007
    Posts
    17

    Pythagoras' theorem

    A rectangle has a length of 21cm and a diagonal measuring 35cm. Find the area of the envelope.

    Can you please also tell me the formula on how to do it please. Thank you
    Follow Math Help Forum on Facebook and Google+

  2. #2
    is up to his old tricks again! Jhevon's Avatar
    Joined
    Feb 2007
    From
    New York, USA
    Posts
    11,663
    Thanks
    3

    Pythagorean's Theorem

    It's the same deal with this question as it was with the other one you posted. We will use Pythagoras' Theorem to find the length of the other side, and then the half-base*height formula for the area of the envelope (which I assume is the "rectangle" mentioned). I hope you realize why we use the two shorter sides to find the area. If not, it's because they are at right angles to each other, so if we places one of the shorter sides on the ground, the other short side would be the vertical height. Anyway, here goes:

    Using a^2=b^2+c^2 (Pythagoras' Theorem: a -hypotenuse, b,c-other two sides)
    => a^2-b^2 = c^2
    using a=35 cm, b=21 cm, we get
    35^2 -21^2 = c^2
    784 = c^2
    squareroot(784) = c
    => c = 28

    For area:
    A = 0.5*base*height
    = 0.5*21*28
    = 294 cm^2
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Super Member
    earboth's Avatar
    Joined
    Jan 2006
    From
    Germany
    Posts
    5,806
    Thanks
    116
    Quote Originally Posted by Jhevon View Post
    ...For area:
    A = 0.5*base*height
    = 0.5*21*28
    = 294 cm^2
    Hello,

    I don't want to pick at you but you calculated here by accident the area of a tringle. The area of a rectangle is:

    A = length * width

    A = 21 * 28 = 588 cm²

    EB
    Follow Math Help Forum on Facebook and Google+

  4. #4
    is up to his old tricks again! Jhevon's Avatar
    Joined
    Feb 2007
    From
    New York, USA
    Posts
    11,663
    Thanks
    3
    You're right! Sorry about that. My bad.
    Follow Math Help Forum on Facebook and Google+

  5. #5
    is up to his old tricks again! Jhevon's Avatar
    Joined
    Feb 2007
    From
    New York, USA
    Posts
    11,663
    Thanks
    3
    Thanks for looking out EB. You should pick on me for stupid mistakes like that. I almost let chhoeuk turn in bad home work! Sorry chhoeuk.
    Follow Math Help Forum on Facebook and Google+

  6. #6
    Forum Admin topsquark's Avatar
    Joined
    Jan 2006
    From
    Wellsville, NY
    Posts
    9,664
    Thanks
    298
    Awards
    1
    Quote Originally Posted by Jhevon View Post
    Thanks for looking out EB. You should pick on me for stupid mistakes like that. I almost let chhoeuk turn in bad home work! Sorry chhoeuk.
    With due respect to incorrect homework, there is no such thing as a "stupid" mistake. Even mistakes can be educational, as I believe this one was.

    -Dan
    Follow Math Help Forum on Facebook and Google+

  7. #7
    is up to his old tricks again! Jhevon's Avatar
    Joined
    Feb 2007
    From
    New York, USA
    Posts
    11,663
    Thanks
    3
    With due respect to incorrect homework, there is no such thing as a "stupid" mistake. Even mistakes can be educational, as I believe this one was.

    -Dan ______

    In general I would agree with you Dan, but not in this case. Mistakes of this nature shouldn't be made by people doing the math courses I'm doing, especially if they decide to help others. You should be responsible enough to be on your p's and q's when others depend on you. So what was the lesson here? ALWAYS READ THE QUESTION THOROUGHLY BEFORE ATTEMPTING TO ANSWER IT! Did someone benifit from the lesson? I'm sure someone did. But that's a lesson that someone like me should know and follow instinctively, and therefore, at least for me, it was a stupid mistake. But like I said, this is an exception, in general I agree that mistakes can be educational--but that doesn't necessarily mean they're not stupid
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Pythagoras' Theorem.
    Posted in the Geometry Forum
    Replies: 2
    Last Post: February 19th 2011, 03:08 AM
  2. Pythagoras Theorem Questions
    Posted in the Geometry Forum
    Replies: 1
    Last Post: June 3rd 2008, 05:43 PM
  3. Pythagoras' Theorem
    Posted in the Geometry Forum
    Replies: 1
    Last Post: February 20th 2007, 01:41 AM
  4. [SOLVED] [SOLVED] Pythagoras theorem
    Posted in the Geometry Forum
    Replies: 5
    Last Post: June 13th 2006, 01:41 PM
  5. pythagoras theorem
    Posted in the Geometry Forum
    Replies: 3
    Last Post: August 1st 2005, 06:17 PM

Search Tags


/mathhelpforum @mathhelpforum