Module 7  Velocity  
Introduction  Lesson 1  Lesson 2  Lesson 3  SelfTest  
Lesson 7.3: Difference Quotients  
The average velocities computed in Lesson 7.2 were examples of righthand Finding Lefthand Difference Quotients A lefthand difference quotient of a function is found by approaching the fixed point from the left.
Determine the lefthand difference quotient of the function stored in y1 by using the points
Instantaneous Velocity Using Lefthand Difference Quotients In Lesson 2 the instantaneous velocity at t = 0.16 was found to be –1.6496 using righthand difference quotients. Would successive lefthand difference quotients produce the same result? 7.3.1 Evaluate lefthand difference quotients over the following intervals: t = 0.159 to t = 0.16 (elapsed time = 0.001 seconds) t = 0.1599 to t = 0.16 (elapsed time = 0.0001 seconds) t = 0.15999 to t = 0.16 (elapsed time = 0.00001 seconds) Click here for the answer. Find the limit of the lefthand difference quotient as the elapsed time approaches 0.
The limit of the lefthand difference quotients produces the same instantaneous velocity as the limit of righthand difference quotients. It can be shown in general that for a function f with a in the domain of f, the instantaneous velocity at the point (a, f(a)) can be found by evaluating either of the following limits. Symmetric Difference Quotients
A symmetric difference quotient is obtained by choosing points evenly spaced on either side of Evaluate the symmetric difference quotient using the points (0.15, y1(0.15)) and (0.17, y1(0.17)).
This result is the same as the instantaneous velocity found earlier. 7.3.2 Find the symmetric difference quotient for the interval from t = 0.159 to t = 0.161. Is the average velocity for this interval also equal to the instantaneous velocity? Click here for the answer. Finding the Limit of the General Symmetric Difference Quotient Values that are equally spaced on either side of t = 0.16 can be written as 0.16 – h and 0.16 + h, where h is some small positive number. The corresponding points are (0.16 – h, y1(0.16 – h)) and (0.16 + h, y1(0.16 + h)), and the general symmetric difference quotient is Find the limit of the general symmetric difference quotient.
For the quadratic function in this lesson, every symmetric difference quotient gives the instantaneous velocity. Clearly then, the limit of the symmetric difference quotients is also the instantaneous velocity. As stated in the following theorem, this property is true for all quadratic functions. Theorem If the function is quadratic, a symmetric difference quotient at a point is equal to the instantaneous velocity at that point. The TI89 can be used to prove the theorem. Proof
Because the symmetric difference quotient is independent of h, It can be shown that is an alternate form which can be used to find instantaneous velocity for any smooth function, so a symmetric difference quotient is equal to the instantaneous velocity for any quadratic function. 

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