Prove that the planes 2x-3y+z=4, x+4y-z=7 and 3x-10y+3z=1 meet in a straight line

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- July 24th 2006, 01:18 AMkingkaisai2Prove-vector
Prove that the planes 2x-3y+z=4, x+4y-z=7 and 3x-10y+3z=1 meet in a straight line

- July 24th 2006, 01:32 AMCaptainBlackQuote:

Originally Posted by**kingkaisai2**

RonL - July 24th 2006, 05:58 AMSoroban
Hello, kingkaisai2!

Quote:

Prove that the planes

We have*no idea*what we're supposed to prove . . .

Its derminant: .

. . so they do__not__intersect at a common point.

- July 25th 2006, 11:16 AMearbothQuote:

Originally Posted by**kingkaisai2**

two planes which are not equal or parallel intersect in a common line. So with your problem you calculate the common line between P1 and P2 and then between P2 and P3. If you get the same line your proof is done:

P1: 2x - 3y = 4 - z

P2: x + 4y = 7 + z

Now calculate for

x= 37/11 - 1/11*z

y= 10/11 + 3/11*z

and set z = 11t

You get the line: (x, y, z) = (37/11, 10/11, 0)+ t*(-1, 3, 11)

P2: x + 4y = 7 + z

P3: 3x - 10y = 1 - 3z

Now calculate for

x = 37/11 - 1/11*z

y = 10/11 + 3/11*z

and set z = 11t

you'll get exactly the same line. So the statement is true.

Greetings

EB - July 25th 2006, 11:26 AMThePerfectHackerQuote:

Originally Posted by**kingkaisai2**

The trick is to use Cramer's rule.

As, Soroban said the determinant of this system is . Therefore, the system has no solutions (planes do not intersect simultaneously) or has infinitely many solutions (lie on a line). But note that,

. Thus, there are infinitely many solutions. Thus, the planes intersect in a line.