Lösung 2.2:2d
Aus Online Mathematik Brückenkurs 1
K (Robot: Automated text replacement (-{{Displayed math +{{Abgesetzte Formel)) |
K (hat „Solution 2.2:2d“ nach „Lösung 2.2:2d“ verschoben: Robot: moved page) |
Version vom 13:55, 22. Okt. 2008
First, we move all the terms over to the left-hand side
| \displaystyle (x^{2}+4x+1)^{2}+3x^{4}-2x^{2}-(2x^{2}+2x+3)^{2}=0\,\textrm{.} |
As the equation stands now, it seems that the best approach for solving the equation is to expand the squares, simplify and see what it leads to.
When the squares are expanded, each term inside a square is multiplied by itself and all other terms
| \displaystyle \begin{align}
(x^{2}+4x+1)^{2} &= (x^{2}+4x+1)(x^{2}+4x+1)\\[5pt] &= x^{2}\cdot x^{2}+x^{2}\cdot 4x+x^{2}\cdot 1+4x\cdot x^{2}+4x\cdot 4x+4x\cdot 1\\[5pt] &\qquad\quad{}+1\cdot x^{2}+1\cdot 4x+1\cdot 1\\[5pt] &= x^{4}+4x^{3}+x^{2}+4x^{3}+16x^{2}+4x+x^{2}+4x+1\\[5pt] &= x^{4}+8x^{3}+18x^{2}+8x+1\,,\\[10pt] (2x^{2}+2x+3)^{2} &= (2x^{2}+2x+3)(2x^{2}+2x+3)\\[5pt] &= 2x^{2}\cdot 2x^{2}+2x^{2}\cdot 2x+2x^{2}\cdot 3+2x\cdot 2x^{2}+2x\cdot 2x\\[5pt] &\qquad\quad{}+2x\cdot 3+3\cdot 2x^{2}+3\cdot 2x+3\cdot 3\\[5pt] &= 4x^{4}+4x^{3}+6x^{2}+4x^{3}+4x^{2}+6x+6x^{2}+6x+9 \\[5pt] &= 4x^{4}+8x^{3}+16x^{2}+12x+9\,\textrm{.} \end{align} |
After we collect together all terms of the same order, the left hand side becomes
| \displaystyle \begin{align}
&(x^{2}+4x+1)^{2}+3x^{4}-2x^{2}-(2x^{2}+2x+3)^{2}\\[5pt] &\qquad{}= (x^{4}+8x^{3}+18x^{2}+8x+1)+3x^{4}-2x^{2}\\[5pt] &\qquad\qquad{}-(4x^{4}+8x^{3}+16x^{2}+12x+9)\\[5pt] &\qquad{}= (x^{4}+3x^{4}-4x^{4})+(8x^{3}-8x^{3})+(18x^{2}-2x^{2}-16x^{2})\\[5pt] &\qquad\qquad{}+(8x-12x)+(1-9)\\[5pt] &\qquad{}= -4x-8\,\textrm{.} \end{align} |
After all simplifications, the equation becomes
| \displaystyle -4x-8=0\quad \Leftrightarrow \quad x=-2\,\textrm{.} |
Finally, we check that \displaystyle x=-2 is the correct answer by substituting \displaystyle x=-2 into the equation
| \displaystyle \begin{align}
\text{LHS} &= \bigl((-2)^{2}+4\cdot(-2)+1\bigr)^{2}+3\cdot (-2)^{4}-2\cdot (-2)^{2}\\[5pt] &= (4-8+1)^{2} + 3\cdot 16 - 2\cdot 4 = (-3)^{2} + 48 - 8 = 9 + 48 - 8 = 49\,,\\[10pt] \text{RHS} &= \bigl(2\cdot(-2)^{2}+2\cdot (-2)+3\bigr)^{2} = (2\cdot 4-4+3)^{2} = 7^{2} = 49\,\textrm{.} \end{align} |
