My Math Forum Inequality help

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 February 23rd, 2017, 04:39 PM #1 Newbie   Joined: Feb 2017 From: usa Posts: 2 Thanks: 0 Inequality help Assume $\displaystyle a,b,c,d$ are positive and $\displaystyle a>b$ as well as $\displaystyle cb$ --- (1) $\displaystyle d>c$ --- (2) so $\displaystyle a+d>b+c$. My teacher said it is wrong, but i am not able to find a counterexample. Appreciate any help.
 February 23rd, 2017, 04:48 PM #2 Math Team   Joined: Jul 2011 From: Texas Posts: 2,785 Thanks: 1434 I hate to say a teacher is wrong, but in this case ... I agree with you. You sure about the starting inequalities? Is there a "problem" to do that you haven't stated completely?
 February 23rd, 2017, 04:54 PM #3 Newbie   Joined: Feb 2017 From: usa Posts: 2 Thanks: 0 Thank you for your answer skeeter. Yes, the starting inequalities are correct, it is just a question I had in general, not a specific problem.
 February 26th, 2017, 02:43 AM #4 Senior Member     Joined: Apr 2014 From: Europa Posts: 575 Thanks: 176 So, we travel in ℝ₊ a > b ⇒ a = b + k d > c ⇒ d = c + m a + d = b+k+c+m ⇒ a + d = (b+c) +(k + m) ⇒ a + d > b + c
 March 23rd, 2017, 11:05 AM #5 Newbie   Joined: Mar 2017 From: VietNam Posts: 7 Thanks: 2 Maybe your teacher say that: it is not equivalent! a>b AND d>c so a+d>b+c, but the counter case is not true! For example: 2+7>3+4, with a=2; b=3; d=7 and c=4.

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