calc 2
Day 2, 2200, jan.13, 2000. Least upper bounds Recall how to describe the real number defined by an infinite decimal in terms of the sequence of finite decimals in its expansion. We said that the real number represented by an infinite decimal, is the smallest real number which is not smaller than any of its finite decimal approximations. This is the first example of the important concept of “least upper bound”. We define an “upper bound” of a set S of real numbers as a number L which is not smaller than any number in S, and a "least upper bound" as an upper bound such that no smaller number is an upper bound. I.e. L is a least upper bound (lub) for the set of numbers in S if and only if: 1) L is an upper bound for numbers in S, i.e. for every number x in S, x ≤ L, and 2) No number smaller than L is an upper bound for all numbers in S, i.e. for any number M < L, there is some number x in S with x > M. Ex: find the l.u.b. of the slopes of all lines joining points of form (1,1) and (x,x2) where x < 1. Solution: By the two point formula, the slope of the line joining (1,1) and (x,x2), is (x^2-1)/(x-1) = x+1, if x < 1. Thus the least number larger than all numbers of form 1+x with x < 1, is 2. I.e. property 1) holds since if x < 1, then 1+x < 2. And property 2) holds since x < 1 implies 1+x < 2. Then if M is any number less than 2, there is an x with x < 1, and 1+x > M. I.e. M < 2 so M-1 < 1, so just take x greater than M-1, and less than 1, i.e. M-1 < x < 1. Then M < 1+x < 2, as desired. Thus the slopes of the secant lines to the curve y = x^2, joining points (1,1) ans (x,x^2) for x<1, have form 1+x, and the smallest number larger than all these slopes shopuld be the slope of the tangent line to the curve y = x^2 at the point (1,1). Namely this slope should be 2. I.e. from looking at the graph of the curve y = x^2, near the point (1,1), we see that the tangent line at (1,1) should have slope larger than the slopes of any of the secant lines joining points (x,x^2), and (1,1), but the slope of these secants becomes arbitrarily near the slopes of the tangent line. Thus the tangent line's slope should be the smallest number larger than the slope of every such secant line, namely 2. Similarly, if we take the point (a,a^2) on the curve y = x^2, the secant lines joining points (a,a^2) and (x,x^2) for x < 2, have slope a+x where x < a. Thus the lub of these slopes is 2a. Hence the slope of the tangent line to y = x^2 at (a,a^2) should be 2a.
Exercise: Show the slope of the tanegnt line to th curve y = x^3 at (1,1) is 3, and at (a,a^3) is 3a^2.