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Algebra Level 3

When x x is real number what is the maximum value of x + 2 2 x 2 + 3 x + 6 ? \frac { x+2 }{ { 2x }^{ 2 }+3x+6 }?


The answer is 0.33.

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6 solutions

U Z
Nov 15, 2014

x + 2 2 x 2 + 3 ( x + 2 ) \frac{x + 2}{2x^{2} + 3(x + 2)}

= x + 2 2 ( x 2 4 ) + 3 ( x + 2 ) + 8 = \frac{x+2}{2(x^{2}-4) + 3(x + 2) + 8}

= 1 2 ( x 2 ) + 3 + 8 x + 2 = \frac{1}{2(x - 2) + 3 + \frac{8}{x + 2}}

= 1 2 ( x + 2 ) + 8 x + 2 5 = \frac{1}{2(x + 2) + \frac{8}{x + 2} - 5}

we will consider maxima for positive real values (can you answer why?)

f ( x ) m i n = 2 ( x + 2 ) + 2 3 x + 2 2 = 2 4 = 8 f(x)_{min} = \frac{2(x + 2) + \frac{2^{3}}{x + 2}}{2} =\sqrt{2^{4}} = 8

Thus maximum = 1 8 5 = 1 3 \frac{1}{8 - 5} = \frac{1}{3}

Parth Kohli
Nov 17, 2014

Let y = x + 2 2 x 2 + 3 x + 6 y = \frac{x+2}{2x^2 + 3x + 6} .

Now,

2 y x 2 + 3 y x + 6 y = x + 2 2yx^2 + 3yx + 6y = x + 2 . 2 y x 2 + ( 3 y 1 ) x + 6 y 2 = 0 \Rightarrow 2yx^2 + (3y -1)x + 6y - 2 = 0

Let this be a quadratic equation in x x . We have to know where this equation has real roots. So we will use the discriminant.

( 3 y 1 ) 2 4 ( 2 y ) ( 6 y 2 ) 0 (3y-1)^2 - 4(2y)(6y -2) \ge 0 .

Solving this inequality, we will get y [ 1 13 , 1 3 ] y \in \left[\frac{1}{13},\frac{1}{3}\right] . So our max is 1 3 \frac{1}{3} .

Nice approach! I like how you turned it into a question about the discriminant of the quadratic equation.

Calvin Lin Staff - 6 years, 6 months ago

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I too agree with Calvin Lin . Very different approach.

Niranjan Khanderia - 6 years, 6 months ago

actually this type of method in coaching cintres

Guru Prasaadh - 6 years, 5 months ago

f ( x ) = x + 2 2 x 2 + 3 x + 6 f ( x ) = 2 x 2 + 3 x + 6 ( x + 2 ) ( 4 x + 3 ) . . . = 0 2 x 2 4 x 2 + 3 x 11 x = 0 x = 0 o r x = 4 x = 0 g i v e M a x . M a x = f ( 0 ) = 1 / 3 1 / 3 f(x)= \frac{x+2}{2x^2+3x+6} \\ f' (x)= \frac{ 2x^2+3x+6-(x+2)(4x+3)}{...} = 0 \\2x^2-4x^2 +3x - 11x =0 \\ x=0~~ or ~~x=-4 \\x=0 ~~give~ Max.~~ Max =f(0)=1/3\\ \boxed{1/3}

Hey how did u give red color in latex please tell me

Mehul Chaturvedi - 6 years, 7 months ago

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In this case, the red color occurred because he had a minor Latex mistake. Specifically, he typed in f^', instead of f'. The ' is already understood to be a superscript, hence the confusion to Latex. I have since fixed this, and his equations appear correctly (eg they were supposed to be fractions).

Otherwise, to add color, you can use \color{red} { x^2 + 2 }, which will give x 2 + 2 \color{#D61F06} { x^2 + 2 } .

Calvin Lin Staff - 6 years, 6 months ago

i did it the same way

ahmed abdo - 6 years, 6 months ago
Jeremiah Jocson
Nov 15, 2014

find the critical pts. then you got the answer, maxima minima if im right :D.....

then zero is the value of x

common sense and logic are needed heheheh

Can you show the steps?

Calvin Lin Staff - 6 years, 6 months ago
Zineb Khatoub
May 4, 2018

f(x)=(x+2)/(2x2+3x+6) we study f we find that f(0) is the max value of f(x) and f(0)= 1/3

Apoorv Shukla
Nov 15, 2014

Find first derivative and equate =0 ; to find values of x, they are 0 and -4. And check the sign of second derivative for values of x. If second derivative is -ve at particular value of x (here it is 0), put this value at place of x to find max. value.

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