Solution 2.3:1d
From Förberedande kurs i matematik 1
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We apply the standard formula for completing the square, | We apply the standard formula for completing the square, | ||
- | + | {{Displayed math||<math>x^{2}+ax = \Bigl(x+\frac{a}{2}\Bigr)^{2} - \Bigl(\frac{a}{2}\Bigr)^{2}\,\textrm{,}</math>}} | |
- | <math>x^{2}+ax=\ | + | |
on our expression and this gives | on our expression and this gives | ||
- | + | {{Displayed math||<math>x^{2}+5x = \Bigl(x+\frac{5}{2}\Bigr)^{2} - \Bigl(\frac{5}{2}\Bigr)^{2} = \Bigl(x+\frac{5}{2}\Bigr)^{2} - \frac{25}{4}\,\textrm{.}</math>}} | |
- | <math>x^{2}+5x=\ | + | |
The whole expression becomes | The whole expression becomes | ||
+ | {{Displayed math||<math>\begin{align} | ||
+ | x^{2}+5x+3 | ||
+ | &= \Bigl(x+\frac{5}{2}\Bigr)^{2} - \frac{25}{4}+3\\[5pt] | ||
+ | &= \Bigl(x+\frac{5}{2}\Bigr)^{2} - \frac{25}{4} + \frac{12}{4}\\[5pt] | ||
+ | &= \Bigl(x+\frac{5}{2}\Bigr)^{2} + \frac{12-25}{4}\\[5pt] | ||
+ | &= \Bigl(x+\frac{5}{2}\Bigr)^{2} - \frac{13}{4}\,\textrm{.} | ||
+ | \end{align}</math>}} | ||
- | + | A quick check shows that we have calculated correctly | |
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- | A quick check shows that we have calculated correctly | + | |
- | + | ||
- | <math>\begin{align} | + | {{Displayed math||<math>\begin{align} |
- | + | \Bigl(x+\frac{5}{2}\Bigr)^{2} - \frac{13}{4} | |
- | & =x^{2}+5x+\frac{12}{4}=x^{2}+5x+3 \\ | + | &= x^{2} + 2\cdot\frac{5}{2}\cdot x + \Bigl(\frac{5}{2}\Bigr)^{2} - \frac{13}{4}\\[5pt] |
- | \end{align}</math> | + | &= x^{2} + 5x + \frac{25}{4} - \frac{13}{4}\\[5pt] |
+ | &= x^{2} + 5x + \frac{12}{4}\\[5pt] | ||
+ | &= x^{2}+5x+3\,\textrm{.} | ||
+ | \end{align}</math>}} |
Current revision
We apply the standard formula for completing the square,
\displaystyle x^{2}+ax = \Bigl(x+\frac{a}{2}\Bigr)^{2} - \Bigl(\frac{a}{2}\Bigr)^{2}\,\textrm{,} |
on our expression and this gives
\displaystyle x^{2}+5x = \Bigl(x+\frac{5}{2}\Bigr)^{2} - \Bigl(\frac{5}{2}\Bigr)^{2} = \Bigl(x+\frac{5}{2}\Bigr)^{2} - \frac{25}{4}\,\textrm{.} |
The whole expression becomes
\displaystyle \begin{align}
x^{2}+5x+3 &= \Bigl(x+\frac{5}{2}\Bigr)^{2} - \frac{25}{4}+3\\[5pt] &= \Bigl(x+\frac{5}{2}\Bigr)^{2} - \frac{25}{4} + \frac{12}{4}\\[5pt] &= \Bigl(x+\frac{5}{2}\Bigr)^{2} + \frac{12-25}{4}\\[5pt] &= \Bigl(x+\frac{5}{2}\Bigr)^{2} - \frac{13}{4}\,\textrm{.} \end{align} |
A quick check shows that we have calculated correctly
\displaystyle \begin{align}
\Bigl(x+\frac{5}{2}\Bigr)^{2} - \frac{13}{4} &= x^{2} + 2\cdot\frac{5}{2}\cdot x + \Bigl(\frac{5}{2}\Bigr)^{2} - \frac{13}{4}\\[5pt] &= x^{2} + 5x + \frac{25}{4} - \frac{13}{4}\\[5pt] &= x^{2} + 5x + \frac{12}{4}\\[5pt] &= x^{2}+5x+3\,\textrm{.} \end{align} |