if x+y+z=10,find the maximum value of 1/x+4/y+9/z?

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justaguide | College Teacher | (Level 2) Distinguished Educator

Posted on

Given that x + y + z = 10 we need to find the maximum value of 1/x + 4/y + 9/z.

We have to assume that x, y and z are distinct and have integral values to be able to derive the maximum value of 1/x + 4/y + 9/z. If they can have any value the expression can have a value that tends to infinity.

Working under the constraints mentioned, we take z as the smallest integer between 0 and 10. That makes z = 1, y is the integer that follows 1 or 2 and x = 10 - 3 = 7.

This gives 1/x + 4/y + 9/z = 1/7 + 4/2 + 9/1 = 1/7 + 2 + 9 = 78/7

If x, y and z have integral values the maximum value of 1/x + 4/y + 9/z = 78/7

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neela | High School Teacher | (Level 3) Valedictorian

Posted on

We choose  x=9.9, y = 1/0.05 and z = 1/0.05, then 1/x+4/y+9/0.05 = 1/10+4/0.05+9/0.05 = 260

We can choose x as near 10 and make y and z  positive  and smaller. Then 1/x+4/y+9/z become larger and unbounded

We make (10-x) -> - 0, y-> +0, z -> +0, then  obviously, 1/x+4/y+9/z  approaches infinity

Therefore the expression 1/x+y/4+9/z has no maximum.

 

 

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