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Mean-Value Theorem (Several Variables)  1 Mean-Value Theorem (Several Variables) THEOREM THE MEAN-VALUE THEOREM (SEVERAL VARIABLES) If  f  is dierentiable at each point of the line segment  ab , then there exists on that line segment a point  c  between  a  and  b  such that f (b) f (a) = ∇f (c) · (b a). Pr oof . As  t  ranges from 0 to 1,  a + t(b − a) traces out the line segment  ab. The idea of the proof is to apply the one-variable mean-value theorem to the function g(t) = f (a + t[b a]), t  ∈  [0, 1]. To show that  g  is dierentiable on the open interval (0 , 1),  we take  t   (0, 1) and form g(t + h) g(t) = f (a + (t + h)[b a]) f (a + t[b a]) = f (a + t[b a] + h[b a]) f (a + t[b a]) = ∇f (a + t[b a]) · h[b a] + o(h[b a]). Since f (a + t[b a]) · h(b a) = [ f (a + t[b a]) · (b a)]h and the  o (h(b a)) term is obviously  o (h), we can write g(t + h) g(t) = [ f (a + t[b a]) · (b a)]h + o(h). Dividing both sides by h , we see that  g  is dierentiable and g (t) = ∇f (a + t[b a]) · (b a). The function g  is clearly continuous at 0 and at 1. Applying the one-variable mean- value theorem to  g, we can conclude that there exists a number  t 0  between 0 and 1 such that g(1) g(0) = g (t 0 )(1 0). Since g (1) = f (b), g(0) = f (a), and  g (t 0 ) = ∇f (a + t 0 [b a]) · (b a), the above gives f (b) f (a) = ∇f (a + t 0 [b a]) · (b a). March 17, 200 1

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