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Math Help - urgent help please... solving simultaneous with powers.

  1. #1
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    Question urgent help please... solving simultaneous with powers.

    If x^2 + y^2 =58 and xy=21, find the vlaue of (x+y)^2 and (x-y)^2.

    if x^2+y^2=1 and xy = 7.5 , find the value of (x^4 -y^4)

    i really don't know how to start , have been thinking for the whole night, still can't do it

    Thank you.
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  2. #2
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    Quote Originally Posted by vivien View Post
    If x^2 + y^2 =58 and xy=21, find the vlaue of (x+y)^2 and (x-y)^2.

    \textcolor{red}{(x+y)^2 = x^2+2xy+y^2 = (x^2+y^2)+2xy = 58 + 2(21) = 100}

    get the idea?

    if x^2+y^2=1 and xy = 7.5 , find the value of (x^4 -y^4)

    i really don't know how to start , have been thinking for the whole night, still can't do it

    Thank you.
    ...
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  3. #3
    Master Of Puppets
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    Quote Originally Posted by vivien View Post
    If x^2 + y^2 =58 and xy=21, find the vlaue of (x+y)^2 and (x-y)^2.
    x^2 + y^2 =58 ....(1)

    xy=21 ....(2)

    From (2) you can say y = \frac{21}{x} and sub this back into (1) giving

    x^2 + \left(\frac{21}{x}\right)^2 =58 ....(1)

    Now solve for x

    I think skeeter's way could be better.
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