algebra
i need help with these problems (either a solution or how to plug them into a graphing calculator):
1) Solve by substitution or elimination
4/x + 1/y + 2/z = 4
2/x + 3/y  1/z = 1
1/x + 1/y + 1/z = 4
2)Solve the system of equations by substitution.
x^2 + y^2 = 25
x  y = 7
3) Solve the system of equations by substitution. Round the answer to the nearest hundredths.
9x^2 + 16y^2 = 140
x^2  4y^2 = 4
4)Solve by elimination:
1/x + 3/y = 7
4/x  2/y = 1
Thanks!

1. add the 2nd and 3rd
> 3/x + 4/y = 5 (#4)
double the 2nd and add to the 1st
> 8/x + 7/y = 6 (#5)
8 times #4 > 24/x + 32/y = 40
3 times #5 > 24/x + 21/y = 18
subtract:
11/y = 22
11 = 22y
y = 1/2
sub back into either #4 or #4 to get x
then back into 1st to get z
2. from the second: x = y+7
into the first: (y+7)^2 + y^2 = 25
2y^2 + 14y + 24 = 0
y^2 + 7y + 12 = 0
(y+3)(y+4) = 0
y = 3 or y = 4
sub both of those into x = y+7 to get their corresponding x values
3. from 2nd > x^2 = 4y^2 + 4
sub into 1st, etc
4. easier than #1
multiply 1st by 4, then subtract the 2ndposted by Reiny
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