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Utilizza un grafico di una retta $y=mx+c$ incluso da Geogebra con coefficienti casuali impostati da NUMBAS. Il grafico contiene anche un punto $P$ di coordinate casuali impostate da NUMBAS. L'esercizio chiede di determinare l'equazione della retta e quelle delle rette parallela e perpendicolare per $P$.

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Il grafico che segue mostra la retta $r$ nel piano cartesiano e il punto $P$ a essa esterno.

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{geogebra_applet('h46bjwqv', defs, [])}

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a) Per determinare l'equazione della retta $r$, osserviamo innanzitutto che, per ogni avanzamento di un'unità verso destra, la retta avanza di $\\simplify[fractionNumbers]{{m}}$ in verticale. Dunque $m = \\simplify[fractionNumbers]{{m}}$. Essendo l'intersezione con l'asse $y$ in $( 0 ; \\var{q})$, si ha $q = \\var{q}$.

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In definitiva

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\\begin{equation} r : \\, y = \\simplify[zeroFactor,fractionNumbers,unitFactor]{{m} x+ {q}} . \\end{equation}

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b) Per determinare le coordinate del punto $P$, è sufficiente osservare il grafico: si ha $P = ({x_P};{y_P})$.

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c) Il fascio (improprio) di rette parallele alla retta data ha equazione $y = \\simplify[zeroFactor,fractionNumbers,unitFactor]{{m} x + q}$. Imponendo il passaggio di una retta del fascio per il punto $P$ otteniamo $q = \\simplify[zeroFactor,fractionNumbers,unitFactor]{{y_P - m*x_P}}$.

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Pertanto

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\\begin{equation} s : \\, y = \\simplify[zeroFactor,fractionNumbers,unitFactor]{{m} x + {y_P - m*x_P}} . \\end{equation}

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d) Il fascio (improprio) di rette perpendicolari alla retta data ha equazione $y = \\simplify[zeroFactor,fractionNumbers,unitFactor]{{- 1/m} x + q}$. Imponendo il passaggio di una retta del fascio per il punto $P$ otteniamo $q = \\simplify[zeroFactor,fractionNumbers,unitFactor]{{y_P + 1/m*x_P}}$.

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Pertanto

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\\begin{equation} t : \\, y = \\simplify[zeroFactor,fractionNumbers,unitFactor]{{-1/m} x + {y_P +1/m*x_P}} . \\end{equation}

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Coefficiente angolare della retta r

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Termine noto (quota) della retta r

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Punto del piano di coordinate intere esterno alla retta r

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Qual è l'equazione della retta $r$?

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$r : \\, y = $ [[0]]

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Quali sono le coordinate del punto $P$?

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$P = $ ([[0]];[[1]])

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Determina l'equazione della retta $s$, parallela a $r$ per il punto $P$

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$s: \\, y = $ [[0]]

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Determina l'equazione della retta $t$, perpendicolare a $r$ per il punto $P$

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$t: \\, y = $ [[0]]

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