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polar coordinates -> cartesian coordinate system ?

• To: mathgroup at smc.vnet.net
• Subject: [mg21394] polar coordinates -> cartesian coordinate system ?
• From: "Guybrush Threepwood" <guybrush_ at gmx.de>
• Date: Mon, 3 Jan 2000 03:12:26 -0500 (EST)
• Organization: bingo e.V. (Buergernetz Ingolstadt e.V.)
• Sender: owner-wri-mathgroup at wolfram.com

Hello

Using the polar coordinate system you can calculate the area with
the formula: 1/2 * Integral [t1,t2] of r^2 d().
Apparently you can convert that to the cartesian coordinate system
so that finally theres a formula like: 1/2 * Integral[t1,t2] of y'*x - x'*y.
Could anyone tell me how to convert the first formula to the second one ?
Btw  in polar coordinates r^2 = x^2 + y^2 !!

ok thanks