The area of the triangle formed by x- and y-intercepts of the parabol y=0.5(x-3)(x+k) is equal to 1.5 square units. Find all possible values of k.
I only got -2 and -1, but it showed it was wrong. Can someone help me figure this out?
The roots are 3 and -k
so the base length is |3 - - k| = |3+K|
The height is given by the y intercept which is |0.5 * -3 * k| = | -1.5k |
\(A=0.5*|3+k|*|-1.5k|\\ A=0.5*1.5*|k|*|3+k|\\ \text{but we want this to equal } 1.5\\ 1.5=0.5*1.5*|k|*|3+k|\\ 1=0.5*|k|*|3+k|\\ 2=|k|*|3+k|\\~\\ If\;\;k<-3\;\;then\\ 2=-k*-(3+k)\\ 2=+k(3+k)\\ ...\\ k=\frac{-3\pm\sqrt{17}}{2}\\ k=k=\frac{-3-\sqrt{17}}{2}\\ k\approx -3.56\)
\(If\;\;-3
\(If\;\;k>0\;\;then\\ 2=k*(3+k)\\ ...\\ k=\frac{-3\pm\sqrt{17}}{2}\\ k=\frac{-3+\sqrt{17}}{2}\\ k\approx 0.56\)
So \(\boxed{k=-2,\;-1, \; \frac{-3-\sqrt{17}}{2},\; \frac{-3+\sqrt{17}}{2}\\ \text{would all work}}\)
It would be a good idea to test each of these for validity.
LaTex:
A=0.5*|3+k|*|-1.5k|\\
A=0.5*1.5*|k|*|3+k|\\
\text{but we want this to equal } 1.5\\
1.5=0.5*1.5*|k|*|3+k|\\
1=0.5*|k|*|3+k|\\
2=|k|*|3+k|\\~\\
If\;\;k<-3\;\;then\\
2=-k*-(3+k)\\
2=+k(3+k)\\
...\\
k=\frac{-3\pm\sqrt{17}}{2}\\
k=k=\frac{-3-\sqrt{17}}{2}\\
k\approx -3.56
If\;\;-3 2=-k*(3+k)\\
-2=k(3+k)\\
...\\
k=\frac{-3\pm\sqrt{17}}{2}\\
k=\frac{-3+\sqrt{17}}{2}\\
k\approx 0.56
I did it graphically and got one answer of k=0.56 (probably correct to 2 decimal places)
The roots are 3 and -k
so the base length is |3 - - k| = |3+K|
The height is given by the y intercept which is |0.5 * -3 * k| = | -1.5k |
\(A=0.5*|3+k|*|-1.5k|\\ A=0.5*1.5*|k|*|3+k|\\ \text{but we want this to equal } 1.5\\ 1.5=0.5*1.5*|k|*|3+k|\\ 1=0.5*|k|*|3+k|\\ 2=|k|*|3+k|\\~\\ If\;\;k<-3\;\;then\\ 2=-k*-(3+k)\\ 2=+k(3+k)\\ ...\\ k=\frac{-3\pm\sqrt{17}}{2}\\ k=k=\frac{-3-\sqrt{17}}{2}\\ k\approx -3.56\)
\(If\;\;-3
\(If\;\;k>0\;\;then\\ 2=k*(3+k)\\ ...\\ k=\frac{-3\pm\sqrt{17}}{2}\\ k=\frac{-3+\sqrt{17}}{2}\\ k\approx 0.56\)
So \(\boxed{k=-2,\;-1, \; \frac{-3-\sqrt{17}}{2},\; \frac{-3+\sqrt{17}}{2}\\ \text{would all work}}\)
It would be a good idea to test each of these for validity.
LaTex:
A=0.5*|3+k|*|-1.5k|\\
A=0.5*1.5*|k|*|3+k|\\
\text{but we want this to equal } 1.5\\
1.5=0.5*1.5*|k|*|3+k|\\
1=0.5*|k|*|3+k|\\
2=|k|*|3+k|\\~\\
If\;\;k<-3\;\;then\\
2=-k*-(3+k)\\
2=+k(3+k)\\
...\\
k=\frac{-3\pm\sqrt{17}}{2}\\
k=k=\frac{-3-\sqrt{17}}{2}\\
k\approx -3.56
If\;\;-3 2=-k*(3+k)\\
-2=k(3+k)\\
...\\
k=\frac{-3\pm\sqrt{17}}{2}\\
k=\frac{-3+\sqrt{17}}{2}\\
k\approx 0.56