CONTOH SOAL 1 IRISAN KERUCUT

$\begin{array}{ll}\\ 1.&\textbf{EBTANAS SMA IPA 1989}\\ &\textrm{Dari suatu elips yang persamaannya}\\ & \displaystyle \frac{(x-5)^{\displaystyle 2}}{25}+\displaystyle \frac{(y+3)^{2}}{16}=1\\ &\textrm{dapat disimpulkan bahwa}\\ &\begin{array}{llll} \textrm{(1)}.&\textrm{Panjang sumbu mayor}=10\\ \textrm{(2)}.&\textrm{Panjang sumbu minor}=10\\ \textrm{(3)}.&\textrm{Koordinat fokusnya adalah}\:\: (8,-3)\:\: \textrm{dan}\:\: (2,-3)\\ \textrm{(4)}.&\textrm{Persamaan sumbu minornya adalah}\:\: x=-3\\  \end{array}\\\\ &\textbf{Pembahasan}:\\ &\textrm{Perhatikan bahwa}\\ &\displaystyle \frac{(x-5)^{\displaystyle 2}}{25}+\displaystyle \frac{(y+3)^{2}}{16}=1\quad \textrm{analog}\quad \displaystyle \frac{(x-p)^{\displaystyle 2}}{a^{\displaystyle 2}}+\displaystyle \frac{(y-q)^{2}}{b^{\displaystyle 2}}=1\\ &\textrm{maka}\\ &\begin{aligned}&\bullet \quad a^{\displaystyle 2}=25\Rightarrow a=5\\ &\bullet \quad b^{\displaystyle 2}=16\Rightarrow b=4\\ &\bullet \quad c^{\displaystyle 2}=a^{\displaystyle 2}-b^{\displaystyle 2}=25-16=9\Rightarrow c=3\\ &\textrm{Sehingga diperoleh}\\ &\ast \quad \textrm{Koordinat pusat}:(p,q)=(5,-3)\\ &(1) \quad \textrm{Panjang sumbu mayor}:2a=2.5=10\quad \textbf{benar}\\ &(2) \quad \textrm{Panjang sumbu minor}:2b=2.4=8\quad \textbf{salah}\\ &(3) \quad \textrm{Koordinat fokus}:(p\pm c,q)=(5\pm 3,-3).\\ &\,\:\quad\quad \textrm{Sehingga}\quad \textrm{F}_{1}(8,-3)\quad \textrm{dan}\quad \textrm{F}_{2}(2,-3)\quad \textbf{benar}\\ &(4) \quad \textrm{Persamaan sumbu minor}:x=5\quad \textbf{salah}\\ \end{aligned} \end{array}$.

$\begin{array}{ll}\\ 2.&\textbf{EBTANAS SMA IPA 1990}\\ &\textrm{Persamaan garis singgung elips}\: \quad 4x^{\displaystyle 2}+y^{\displaystyle 2}=4\\ &\textrm{yang sejajar dengan garis}\quad 2x-y-5=0\:\: \textrm{adalah}\:....\\ &\begin{array}{llll}\\ \textrm{A}.&\displaystyle y=-2x\pm 2\sqrt{2}\\ \textrm{B}.&\displaystyle y=\displaystyle -\frac{1}{2}x\pm 2\sqrt{2}\\ \textrm{C}.&\displaystyle y=-2x\pm 2\\ \textrm{D}.&\displaystyle y=2x\pm +2\\ \textrm{E}.&\displaystyle y=2x\pm 2\sqrt{2} \end{array}\\\\ &\textrm{Jawab}:\quad \textbf{E}\\ &\textrm{Perhatikan bahwa garis}\quad 2x-y-5=0\\ &y=2x-5\quad \textrm{analog}\quad y =mx+c\Rightarrow m=2\\ &\textrm{dan}\quad 4x^{\displaystyle 2}+y^{\displaystyle 2}=4\Rightarrow \displaystyle \frac{4x^{\displaystyle 2}}{4}+\displaystyle \frac{y^{\displaystyle 2}}{4}=1\\ &\bullet \quad a^{\displaystyle 2}=1,\quad b^{\displaystyle 2}=4,\quad \textrm{dan}\quad m=2\\ &\textrm{maka persamaan garis singgungnya}:\\ &y=mx\pm \sqrt{a^{\displaystyle 2}m^{\displaystyle 2}+b^{\displaystyle 2}}\\ &\Leftrightarrow y=2x\pm \sqrt{1^{\displaystyle 2}.2^{\displaystyle 2}+2^{\displaystyle 2}}\\ &\Leftrightarrow y=2x\pm \sqrt{8}\\ &\Leftrightarrow y=2x\pm 2\sqrt{2} \end{array}$.