Inequality Chain

Algebra Level 2

Let a , b , c , d , a, b, c, d, and e e be real numbers such that

a + b < c + d a+b<c+d

b + c < d + e b+c<d+e

c + d < e + a c+d<e+a

d + e < a + b d+e<a+b .

Which among a , b , c , d , e a,b,c,d,e is the largest and smallest?

Put your answer in the form (largest, smallest). For example, if you think the largest is c c and the smallest is e e , then your answer should be ( c , e ) (c, e) .

Cannot be determined ( c , b ) (c, b) ( a , b ) (a, b) ( c , d ) (c, d) ( a , e ) (a, e) ( e , a ) (e, a)

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3 solutions

Tan Wei Xin
Aug 30, 2015

From the inequalities, we can form a "chain" of inequalities b + c < d + e < a + b < c + d < e + a b+c<d+e<a+b<c+d<e+a .

From the first 3 terms, we can deduce c < a , e < c , and b < e c<a,e<c, \text{ and } b<e by comparing each individual term to the second term to the right of it ( b + c < a + b , d + e < c + d , a + b < e + a ) (b+c<a+b, \hspace{.3cm} d+e<c+d, \hspace{.3cm} a+b<e+a) .

Once again, we can form the "chain" of inequalities b < e < c < a b<e<c<a .

Because e < c e<c , and b + c < d + e b+c<d+e , then d d must be larger than b b , else we will get a contradiction.

Similarly, because e < c e<c , and c + d < e + a c+d<e+a , then d d must be smaller than a a , else we will get a contradiction.

From the above 2 statements, we can deduce b < d < a b<d<a .

Beacause b < d < a b<d<a , and b < e < c < a b<e<c<a , in all possible cases or "chain", b b is always the smallest, a a is always the greatest.

Excellent. well done, and thanks :)

Պոոռնապռագնյա ՊՌ - 5 years, 9 months ago

Well explained.

Bob Smiley - 4 years, 6 months ago
Owen Leong
Nov 3, 2015

b and c appeared most frequently on the LHS and least frequently on the RHS. So either b or c is smallest. Similarly, either a or e is the largest. The only answer which fits is (a, b). (This won't work if more options were given but a quick solution for this question nonetheless)

Hadia Qadir
Sep 6, 2015

mallest is b and largest is a. Solution: From all the above we get: b + c < d + e < a + b < c + d < e + a. We can say: c < a, b < d, e < c, d < a, b < e. Therefore: b < a, c, d, e which means b is the smallest. And a > b, c, d, e which means a is the largest

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