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What is number of roots of equation 4^x+2^x=272?

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greenbel

Posted July 22, 2013 at 5:13 PM via web

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What is number of roots of equation 4^x+2^x=272?

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sciencesolve | Teacher | (Level 3) Educator Emeritus

Posted July 22, 2013 at 5:24 PM (Answer #1)

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You should come up with the following substitution, such that:

`2^x = t`

Replacing the variable in equation, yields:

`t^2 + t = 272 => t^2 + t - 272 = 0`

Using quadratic formula, yields:

`t_(1,2) = (-1+-sqrt(1 + 1088))/2 => t_(1,2) = (-1+-33)/2`

`t_1 = (-1+33)/2 => t_1 = 16`

`t_2 = (-1-33)/2 => t_2 = -17`

You need to solve for x the equations `2^x = t_1` and `2^x = t_2` , such that:

`2^x = t_1 => 2^x = 16 => 2^x = 2^4 `

Equating the powers yields:

`x = 4`

`2^x = t_2 => 2^x = -17` invalid because `2^x > 0`

Hence, evaluating the number of solutions to the given equation, yields one solution, `x = 4.`

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aruv | High School Teacher

Posted July 22, 2013 at 5:31 PM (Answer #2)

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`4^x+2^x=272`

rewrite equation as

`(2^2)^x+2^x-272=0`

`(2^x)^2+2^x-272=0`

Let `2^x=y` then above equation reduces to

`y^2+y-272=0`

which is quadratic equation.So it has two roots which  may be real /complex.

`y^2+17y-16y-272=0`

`(y+17)(y-16)=0`

`y=-17`

`or y=16`

i.e.

`2^x=-17`  which is not possible for any value of x.

and

`2^x=16=2^4`

`x=4`

Thus it has one real root and other root not possible.

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degeneratecircle | High School Teacher | (Level 2) Associate Educator

Posted July 22, 2013 at 10:39 PM (Answer #3)

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Another method is to use the fact that `f(x)=4^x+2^x` is an increasing function. If you're not expected to know calculus, then you can probably assume this, maybe by appealing to the graph.

If you know calculus, then this isn't too difficult to prove using derivatives.

Once you know it's an increasing function, then there can be at most one point where `f(x)=272.` Again by using calculus (specifically the Intermediate Value Theorem), or by trying small numbers until one works, we can say that there is exactly one value of `x` for which `4^x+2^x=272.`

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