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Lösung 3.1:4e

Aus Online Mathematik Brückenkurs 2

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K (Robot: Automated text replacement (-{{Displayed math +{{Abgesetzte Formel))
Zeile 1: Zeile 1:
If we multiply both sides by <math>z+i</math>, we avoid having <math>z</math> in the denominator,
If we multiply both sides by <math>z+i</math>, we avoid having <math>z</math> in the denominator,
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{{Displayed math||<math>iz+1=(3+i)(z+i)\,\textrm{.}</math>}}
+
{{Abgesetzte Formel||<math>iz+1=(3+i)(z+i)\,\textrm{.}</math>}}
At the same time, this means that we are now working with a new equation which is not necessarily entirely equivalent with the original equation. Should the new equation show itself to have <math>z=-i</math> as a solution, then we must ignore that solution, because our initial equation cannot possibly have <math>z=-i</math> as a solution (the denominator of the left-hand side becomes zero).
At the same time, this means that we are now working with a new equation which is not necessarily entirely equivalent with the original equation. Should the new equation show itself to have <math>z=-i</math> as a solution, then we must ignore that solution, because our initial equation cannot possibly have <math>z=-i</math> as a solution (the denominator of the left-hand side becomes zero).
Zeile 7: Zeile 7:
We expand the right-hand side in the new equation,
We expand the right-hand side in the new equation,
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{{Displayed math||<math>iz+1 = 3z+3i+iz-1\,,</math>}}
+
{{Abgesetzte Formel||<math>iz+1 = 3z+3i+iz-1\,,</math>}}
and move all the terms in <math>z</math> over to the left-hand side and the constants to the right-hand side,
and move all the terms in <math>z</math> over to the left-hand side and the constants to the right-hand side,
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{{Displayed math||<math>\begin{align}
+
{{Abgesetzte Formel||<math>\begin{align}
iz-3z-iz &= 3i-1-1\,,\\[5pt]
iz-3z-iz &= 3i-1-1\,,\\[5pt]
-3z &= -2+3i\,\textrm{.}
-3z &= -2+3i\,\textrm{.}
Zeile 18: Zeile 18:
Then, we obtain
Then, we obtain
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{{Displayed math||<math>z = \frac{-2+3i}{-3} = \frac{2}{3}-i\,\textrm{.}</math>}}
+
{{Abgesetzte Formel||<math>z = \frac{-2+3i}{-3} = \frac{2}{3}-i\,\textrm{.}</math>}}
It is a little troublesome to divide two complex numbers, so we will therefore not check whether <math>z=\tfrac{2}{3}-i</math> is a solution to the original equation, but satisfy ourselves with substituting into the equation <math>iz+1=(3+i)(z+i)</math>,
It is a little troublesome to divide two complex numbers, so we will therefore not check whether <math>z=\tfrac{2}{3}-i</math> is a solution to the original equation, but satisfy ourselves with substituting into the equation <math>iz+1=(3+i)(z+i)</math>,
-
{{Displayed math||<math>\begin{align}
+
{{Abgesetzte Formel||<math>\begin{align}
\text{LHS} &= iz+1 = i(\tfrac{2}{3}-i)+1 = \tfrac{2}{3}\cdot i+1+1 = 2+\tfrac{2}{3}i,\\[5pt]
\text{LHS} &= iz+1 = i(\tfrac{2}{3}-i)+1 = \tfrac{2}{3}\cdot i+1+1 = 2+\tfrac{2}{3}i,\\[5pt]
\text{RHS} &= (3+i)(z+i) = (3+i)(\tfrac{2}{3}-i+i) = (3+i)\tfrac{2}{3} = 2+\tfrac{2}{3}i\,\textrm{.}
\text{RHS} &= (3+i)(z+i) = (3+i)(\tfrac{2}{3}-i+i) = (3+i)\tfrac{2}{3} = 2+\tfrac{2}{3}i\,\textrm{.}
\end{align}</math>}}
\end{align}</math>}}

Version vom 13:07, 10. Mär. 2009

If we multiply both sides by z+i, we avoid having z in the denominator,

iz+1=(3+i)(z+i).

At the same time, this means that we are now working with a new equation which is not necessarily entirely equivalent with the original equation. Should the new equation show itself to have z=−i as a solution, then we must ignore that solution, because our initial equation cannot possibly have z=−i as a solution (the denominator of the left-hand side becomes zero).

We expand the right-hand side in the new equation,

iz+1=3z+3i+iz−1

and move all the terms in z over to the left-hand side and the constants to the right-hand side,

iz−3z−iz−3z=3i−1−1=−2+3i.

Then, we obtain

z=−3−2+3i=32−i.

It is a little troublesome to divide two complex numbers, so we will therefore not check whether z=32−i is a solution to the original equation, but satisfy ourselves with substituting into the equation iz+1=(3+i)(z+i),

LHSRHS=iz+1=i(32−i)+1=32i+1+1=2+32i=(3+i)(z+i)=(3+i)(32−i+i)=(3+i)32=2+32i.
Von „http://wiki.math.se/wikis/2009/bridgecourse2-TU-Berlin/index.php/L%C3%B6sung_3.1:4e“