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.

Printable View

- Dec 16th 2009, 03:56 PMvivienurgent 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. - Dec 16th 2009, 04:01 PMskeeter
- Dec 16th 2009, 04:04 PMpickslides
$\displaystyle x^2 + y^2 =58 $ ....(1)

$\displaystyle xy=21$ ....(2)

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

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

Now solve for x

I think skeeter's way could be better.