How to find `lim_(x-gtoo)(1+2x-3x^5)` , ?  

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lemjay's profile pic

lemjay | High School Teacher | (Level 2) Senior Educator

Posted on

We should consider the following properties of limits.

>>` lim_(x-gtoo) = cx^n = oo`            

>> `lim_(x-gtgtoo) 1/x^n = 0`

>> If `lim_(x-gtc) f(x) ``= oo ` and  ` lim_(x-gtc) = L`  (where L<0),

     then  ` lim_(x-gtc) [f(x) g(x)]= -oo` .


To evaluate the limit above, factor out the x with largest power.

`lim_(x-gtoo) x^5(1/x^5 +2/x^4 - 3)`

Then, take the limit of each factor.

`lim_(x-gtoo) x^5 = oo`       and     `lim_(x-gtoo) (1/x^5+2/x^4-3) = 0+0-3 = -3`


Hence, the value of the limit is:

`lim_(x-gtoo) (1+x^2-3x^5) = -oo`


tonys538's profile pic

tonys538 | Student, Undergraduate | (Level 1) Valedictorian

Posted on

The limit `lim_(x->oo) 1 + 2x - 3x^5` has to be determined.

Notice the three terms in the expression 1 + 2x - 3x^5.

As x tends to `oo` , 1 does not change, 2x increases and 3x^5 also increases. But the rate at which 3x^5 increases is faster than that at which 2x increases as x has been multiplied by a larger number and also because x has been raised to a higher power.

As a result -3x^5, approaches `-oo` at a faster rate than at which 2x approaches `oo` .

This gives the limit `lim_(x->oo) 1 + 2x - 3x^5 = -oo` .

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