Chapter 3: Applications of Differentiation
Section 3.1: Tangent and Normal Lines
Example 3.1.2
Which point on the graph of gx=7 x+9−x2 is closest to the point 3,10? What is that minimal distance?
Solution
Visualization
Define gx
Control-drag gx=…
Context Panel: Assign Function
gx=7 x+9−x2→assign as functiong
Define dx, the distance from 3,10 to x,gx
Write dx=…
dx=3−x2+10− gx2→assign as functiond
Graph dx
Figure 3.1.2(a) contains a graph of dx, the function giving the distance from the fixed point 3,10 to x,gx, the generic point on the graph of gx.
The smallest value for dx occurs in the interval 0,1/2.
The minimum distance is slightly less than 3.
Figure 3.1.2(a) Graph of dx
Animation
The animation in Figure 3.1.2(b) is controlled by the slider that sets the value of the x-coordinate of the moving green dot in the figure. This dot marks the point on the graph of gx corresponding to that value of x. The black dot marks the fixed point 3,10.
x = =
The corresponding y-coordinate is
y =
and the length of the red segment (giving d, the distance between the points) is
d =
Figure 3.1.2(b) Slider-controlled animation
Move the slider to determine the minimum value of d.
Computation
The line from the fixed point 3,10 to x,gx on the graph of g must be orthogonal to the graph, that is, orthogonal to the tangent line at x,gx. Hence, this line must be coincident with the normal line through x,gx.
Alternatively, the slope of the segment connecting 3,10 with x,gx and the slope of the normal line at x,gx) must agree.
Solve for xmin
Assuming gx and dx have already been defined, write the equation expressing the equality of slopes.
Press the Enter key.
Context Panel: Solve≻ Numerically Solve from point≻x=1
−1g′x=gx−10x−3
−17−2⁢x=7⁢x−1−x2x−3
→solve
0.2097141098
Obtain dmin=gxmin
Evaluate dx at xmin. Context Panel: Evaluate and Display Inline
d = 2.822319482
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