Q. The parabolas y=x2+1 and y=19−(x−a)2 are tangent to one another and tangent to the line y=mx+b. Find m and b.
Find Derivatives: We know that if two curves are tangent to each other at a point, they have the same slope at that point. The slope of a curve given by y=f(x) is the derivative f′(x). Let's find the derivatives of the given parabolas.First parabola: y=x2+1Derivative: dxdy=2xSecond parabola: y=19−(x−a)2Derivative: dxdy=−2(x−a)
Set Equal Slopes: Since the parabolas are tangent to each other, their slopes are equal at the point of tangency. Therefore, we can set their derivatives equal to each other to find the x-coordinate of the point of tangency.2x=−2(x−a)Solving for x gives us:2x=−2x+2a4x=2ax=2a
Calculate Tangency Point: Now we need to find the y-coordinate of the point of tangency by substituting x=2a into either of the original equations. We'll use the first parabola's equation.y=(2a)2+1y=4a2+1
Determine Line Equation: The line y=mx+b is tangent to both parabolas, which means it has the same slope as the parabolas at the point of tangency. Therefore, m must be equal to the derivative of the parabolas at the point of tangency.m=2x=2(a/2)=a
Find Value of a: To find b, we need to use the point of tangency (2a,4a2+1) and the slope m=a in the equation of the line y=mx+b.4a2+1=a(2a)+bSolving for b gives us:b=4a2+1−4a2b=1
Solve Quadratic Equation: We have found that m=a and b=1. However, we still need to determine the value of a. To do this, we need to use the fact that the second parabola is also tangent to the line y=mx+b. We substitute y=mx+b into the second parabola's equation and solve for a.19−(x−a)2=ax+1
Calculate Discriminant: Expanding the equation and collecting like terms gives us:19−x2+2ax−a2=ax+1−x2+(2a−a)x+(19−a2−1)=0−x2+ax+(18−a2)=0
Solve for a: Since the line is tangent to the parabola, this quadratic equation must have exactly one solution for x. This means the discriminant of the quadratic equation must be zero.Discriminant: (a)2−4(−1)(18−a2)=0a2+4(18−a2)=0a2+72−4a2=0−3a2+72=0
Determine Possible Slopes: Solving for a gives us:−3a2=−72a2=24a=±24a=±26Since m=a, we have two possible values for m: m=26 or m=−26.
Determine Possible Slopes: Solving for a gives us:−3a2=−72a2=24a=±24a=±26Since m=a, we have two possible values for m: m=26 or m=−26.We have two possible slopes for the line, m=26 or m=−26, and the y-interceptb=1. These are the values of m and b for the line tangent to both parabolas.
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