Q. The equation y=x2+6x+b is graphed in the xy-plane. If the vertex of the graph of the equation is at (−3,0), what is the value of b ?
Identify vertex form: Identify the vertex form of a parabola.The vertex form of a parabola is y=a(x−h)2+k, where (h,k) is the vertex of the parabola.
Write equation with given vertex: Use the given vertex (−3,0) to write the equation in vertex form.Since the vertex is (−3,0), we have h=−3 and k=0. Thus, the vertex form of the equation is y=a(x−(−3))2+0, which simplifies to y=a(x+3)2.
Expand vertex form: Expand the vertex form to compare it with the given equation.Expanding y=a(x+3)2 gives us y=a(x2+6x+9). Since the given equation is y=x2+6x+b, we can compare the coefficients to find the value of a and b.
Compare coefficients for a: Compare the coefficients of x2 and x to find the value of a.The coefficient of x2 in both the expanded vertex form and the given equation is 1, so a=1. The coefficient of x is also the same (6), which confirms that a=1 is correct.
Determine value of b: Determine the value of b by comparing the constant terms.In the expanded vertex form, the constant term is 9a, which is 9 since a=1. For the vertex form to match the given equation, the constant term must also be b. Therefore, b=9.
Verify vertex with b=9: Verify that the vertex (−3,0) is correct with b=9.Substitute x=−3 into the given equation y=x2+6x+9 to check if y equals 0. y=(−3)2+6(−3)+9=9−18+9=0. This confirms that the vertex is indeed (−3,0) when b=9.
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