The graph of a polynomial function approaches –infinity as x approaches –infinity, and approaches + infinity as x approaches + infinity. Which could be the degree and leading coefficient of this function?

Respuesta :

Here are the four possible combinations for the parity of the degree and the sign of the leading coefficient:

Case 1: odd degree and positive leading coefficient

If the lead coefficient is positive and the degree of polyonimial is odd, then the limits will be

[tex]\displaystyle \lim_{x\to-\infty}p(x)=-\infty,\quad \lim_{x\to\infty}p(x)=\infty[/tex]

In fact, odd powers maintain the sign (for example, [tex](-2)^3=-8[/tex] and [tex]3^3=27[/tex]. So, a very large number with an odd exponent will be even larger, and will keep the same sign.

Case 2: odd degree and negative leading coefficient

In this case, everything we said about the previous case still holds, but the negative coefficient swaps all the sign. So, we have

[tex]\displaystyle \lim_{x\to-\infty}p(x)=\infty,\quad \lim_{x\to\infty}p(x)=-\infty[/tex]

Case 3: even degree and positive leading coefficient

Even exponents always return a positive value: if the input was positive the result remains positive (e.g. [tex]3^2=9[/tex]), and if the input was negative, the result becomes positive (e.g. [tex](-2)^4=+16[/tex]).

So, it doesn't matter if [tex]x[/tex] gets bigger and bigger in the positive or negative direction, the result will be positive nevertheless:

[tex]\displaystyle \lim_{x\to-\infty}p(x)=\infty,\quad \lim_{x\to\infty}p(x)=\infty[/tex]

Case 4: even degree and negative leading coefficient

Obviosuly, in this case, the negative leading coefficient causes all the signs to swap:

[tex]\displaystyle \lim_{x\to-\infty}p(x)=-\infty,\quad \lim_{x\to\infty}p(x)=-\infty[/tex]

PS: Throughout all the answer I only looked at the leading coefficient, because it's the one that decides the end behaviour of the polynomial. In fact, we have

[tex]\displaystyle \lim_{x\to\pm\infty}p(x)=\lim_{x\to\pm\infty} a_nx^n+a_{n-1}x^{n-1}+\ldots+a_2x^2+a_1x+a_0[/tex]

If we factor the leading power we have

[tex]\displaystyle\lim_{x\to\pm\infty} x^n\left(a_n+\dfrac{a_{n-1}}{x}+\ldots+\dfrac{a_2}{x^{n-2}}+\dfrac{a_1}{x^{n-1}}+\dfrac{a_0}{x^n}\right)[/tex]

Since [tex]x[/tex] is going to infinity, all the terms with [tex]x[/tex] at the denominator will vanish in the limit, and so we have

[tex]\displaystyle\lim_{x\to\pm\infty} x^n\left(a_n+\dfrac{a_{n-1}}{x}+\ldots+\dfrac{a_2}{x^{n-2}}+\dfrac{a_1}{x^{n-1}}+\dfrac{a_0}{x^n}\right)=\displaystyle\lim_{x\to\pm\infty} a_nx^n[/tex]

And this limit depends only on the sign of the leading coefficient and the parity of the exponent, as we discussed above.

Answer:

C. degree 5, leading coefficient 1

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Step-by-step explanation: