Q. A parabola graphed in the xy-plane has equation y=−3x2+9x−2. What is the y-coordinate of the vertex of the parabola?
Write quadratic equation: Write down the given quadratic equation.The given quadratic equation is y=−3x2+9x−2.
Convert to vertex form: Convert the quadratic equation into vertex form.The vertex form of a quadratic equation is y=a(x−h)2+k, where (h,k) is the vertex of the parabola.To convert the given equation into vertex form, we need to complete the square.
Factor out x terms: Factor out the coefficient of x2 from the x terms.Factor out −3 from −3x2 and 9x.y=−3(x2−3x)−2
Find (b/2)2: Find the value of (b/2)2 for x2−3x. The coefficient of x is −3. (−3/2)2=(−1.5)2=2.25
Complete the square: Add and subtract (2b)2 inside the parentheses to complete the square.y=−3(x2−3x+2.25−2.25)−2y=−3((x2−3x+2.25)−2.25)−2
Rewrite as perfect square trinomial: Rewrite the equation as a perfect square trinomial.y=−3((x−1.5)2−2.25)−2
Distribute and simplify: Distribute the −3 and simplify the equation.y=−3(x−1.5)2+6.75−2y=−3(x−1.5)2+4.75
Identify the vertex: Identify the vertex of the parabola.The vertex form of the equation is now y=−3(x−1.5)2+4.75, so the vertex is at (h,k)=(1.5,4.75).The y-coordinate of the vertex is 4.75.
More problems from Write a quadratic function in vertex form