Graph of Quadratic Function/Solving Quadratic Equation for (a=-2,b=-6,c=-30)

Let us consider the following quadratic function: f(x) = -2x2 -6x -30

The solutions of the quadratic equation -2x2 -6x -30 = 0 correspond to the zeros or the roots of the function f(x) = -2x2 -6x -30. 
These are the points where the graph of f(x) cuts the x-axis. There are no such points or roots in the graph of this function over here because the roots are complex, not real. The graph cuts the Y-axis at -30.

The Discriminant and Roots of the Quadratic Equation -2x2 -6x -30 = 0

The standard form of a quadratic equation is ax2 + bx + c = 0, where "a" does not equal 0. Note that if a = 0, the x2 term would disappear and the equation would be linear.
Looking at the given quadratic function  a = -2, b = -6, c = -30. 
The discriminant D = b2 - 4ac = -62 - 4 * (-2) * (-30) = -204.0
The roots of the equation are complex conjugates and they are  (-b - √D)/2a and (-b + √D)/2a 
= (-(-6) - √-204.0)/(2(-2)) and (-(-6) + √-204.0)/(2(-2))
-1.5+3i and -1.5-3i 

The discriminant is negative. Hence, the roots are imaginary. The quadratic curve does not cut the X-axis. The curve lies below the X-axis.


Graph of y = f(x) = -2x2 -6x -30
Quadratic Polynomial Parabola


Geometric and Graphical interpretation: Curve Sketching


The function f(x) = -2x2 -6x -30 is the quadratic function. The graph of any quadratic function has the same general shape. This shape is called a parabola. The location and size of the parabola, and how it opens, depend on the values of coefficients in the function. This parabola has a maxima point and opens downwards. The graph of the parabola is symmetric with respect to the vertical line passing through the vertex. 

The x-coordinate of the vertex will be located at x = (-b/2a) = (-(-6))/(2*-2) = -1.5, and the y-coordinate of the vertex is -25.5 which we obtain by substituting the value x = -1.5 in -2x2 -6x -30. This is the maxima value attained by the quadratic function f(x). The derivative of the function is 0 at this point. This point is a turning point or a stationary point.

The vertex is the lowest point on the parabola if the parabola opens upward (coefficient a > 0) and is the highest point on the parabola if the parabola opens downward (coefficient a < 0)


Check the plot of another quadratic curve here.

Here's another quadratic curve here.
Many of these concepts are a part of the Grade 9,10,11,12 (High School) Mathematics syllabus of the UK GCSE curriculum, Common Core Standards in the US, ICSE/CBSE/SSC syllabus in Indian high schools. You may check out our free and printable worksheets for Common Core and GCSE.



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