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March 21st, 2018, 07:00 AM  #1 
Senior Member Joined: Nov 2015 From: hyderabad Posts: 232 Thanks: 2  Range of Continuous & Differentiable Sets
Let $f:R > R$ and $f(x) = x1+x2$. Let $S1 = ${$x f$is continuous at $x$} and $S2 = ${$x f$ is differentiable at $x$} then a) $S1 = R , S2=R$ b) $S1 = R  ${$1,2$}$, S2 = R$ c) $S1 = R , S2 = R  ${$1,2$} d) $S1 = R  ${$1,2$}$, S2 = R  ${$1,2$} Which of the following is true ? I have seen individual questions like $f(x) = x1$ where it is not continuous at $x=1$. Simillarly, $f(x) = x2$ where it is not continuous at $x=2$. 
March 21st, 2018, 10:01 AM  #2 
Math Team Joined: Jul 2011 From: Texas Posts: 2,781 Thanks: 1432 
for $x \ge 2$ ... $f(x) = (x1)+(x2) = 2x3$ $f'(x) = 2$ for $1 < x < 2$ ... $f(x) = (x1)  (x2) = 1$ $f'(x) = 0$ for $x \le 1$ ... $f(x) = (x1)(x2) = 2x+3$ $f'(x) = 2$ $f(x)$ is continuous over $\mathbb{R}$ $f(x)$ is differentiable over $\mathbb{R}  \{1,2\}$ 
March 22nd, 2018, 05:13 AM  #3 
Senior Member Joined: Nov 2015 From: hyderabad Posts: 232 Thanks: 2 
Thank you so much. We have to redefine the function to check continuity & differentiability since the limits have not given. And after checking continuity & differentiability over 1 & 2, it has given the clear answer.

March 24th, 2018, 07:14 AM  #4  
Math Team Joined: Jan 2015 From: Alabama Posts: 3,261 Thanks: 894  Quote:
If x< 1 then both x 1 and x 2 are negative so f(x)= (x 1) (x 2)= 2x+ 3. If 1< x< 2 then x 1 is positive and x 2 is still negative so f(x)= x 1 (x 2)= 1. If x> 2 then both x 1 and x 2 are positive so f(x)= x 1+ x 2= 2x 3. The limit, as x goes to 1 "from below", is 2(1)+ 3= 1 and "from above" is 1 so f is continuous there. The derivative of f, for x< 1,is just the slope of 2x+ 3, 2. For 1< x< 2, f is constant so the derivative is 0. Those are not the same so f is not differentiable at x= 1. The limit,as x goes to 2 "from below", is 1 and "from above" is 2(2) 3= 1 so the function is continuous at x= 2. The derivative of x, for 1< x< 2, is 0. The derivative of f, for x> 2, is 2. Those are not the same so f is not differentiable at x= 2. (Note: the derivative of a continuous function is NOT necessarily continuous itself but must satisfy the "intermediate value property" so that the two right and left sided slopes not being the same is sufficient to say that the derivative does not exist.)  

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