Increasing Variables

Calculus Level 2

It is given that S = a b + c d + 1 e S=\frac{a}{b} + \frac{c}{d} + \frac{1}{e} , where 0 < a < b < c < d < e 0 < a < b < c < d < e .

For which variable, would adding 1 to the value of it, increase S S by the greatest amount?

d b a c

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4 solutions

Dpk ­
Mar 16, 2014

It's\quad pretty\quad obvious\quad that\quad b,d,\quad and\quad e\quad are\quad wrong\quad choices,\\ \\ so\quad it's\quad only\quad a\quad choice\quad between\quad a\quad and\quad c.\\ \\ Adding\quad 1\quad to\quad a\quad adds\quad \frac { 1 }{ b } \quad to\quad S,\\ \\ while\quad adding\quad 1\quad to\quad c\quad adds\quad \frac { 1 }{ d } \quad to\quad S.\\ \\ since\quad b\quad <\quad d,\quad if\quad we\quad get\quad the\quad cross\quad product\\ \\ we\quad find\quad out\quad that\quad \frac { 1 }{ b } \quad >\quad \frac { 1 }{ d } ,\\ \\ therefore\quad adding\quad to\quad a\quad would\quad give\quad the\quad largest\quad increase\quad in\quad S.

cool

Urza Hale - 7 years, 2 months ago

a

Iitian Sotam - 7 years, 2 months ago

assume a=1 b=2 c=3 d=4 e=5 according to the given condition and just 1 to each of them and sustitute in the equation you will get the answer.

Saurav Sharma - 7 years, 1 month ago

the main thing is to observe this question carefully

Kanishk Singh - 7 years, 1 month ago
Al Imran
Mar 14, 2014

since, a < b < c < d < e; therefore b < d < e => 1/b>1/d>1/e. so, a

a=c

Kiran Chudhry - 7 years, 2 months ago
Prashant Kumar
Mar 29, 2014

add 1 to the smallest number which is in numerator

Julieta Ramos
Mar 25, 2014

The greater the numerator, the greater is the number

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