Can we obtain the polynomial \(h(x)=x\) by adding, subtracting, or multiplying the polynomials \(f(x)=x^2+x\) and \(g(x)=x^2+2\)?
The topic of this problem sheet will be polynomials. Before we dive into the examples, let’s recap a few key concepts.
A polynomial in \(x\) is an expression formed by adding or subtracting monomials, which are terms of the form \(a x^n\), where \(a\) is a number called a coefficient, and \(n\) is a whole number (non-negative integer). Here, \(x\) is a variable that may represent a number.
We can perform several familiar operations on polynomials, which you may have seen before:
Addition and subtraction: We add or subtract polynomials by looking at each power of \(x\) and adding or subtracting the corresponding coefficients. For example, if \[f(x) = x^4 + 3x - 1 \quad \text{and} \quad g(x) = x^3 + 2x + 5,\] then \[f(x) - g(x) = x^4 - x^3 + x - 6.\]
Multiplication: We use the distributive property, which means that every term in the first polynomial is multiplied by every term in the second polynomial. For example, if \[f(x) = x^2 + x + 1 \quad \text{and} \quad g(x) = x - 1,\] then \[f(x) g(x) = (x^2 + x + 1)(x - 1) = x^3 + x^2 + x - x^2 - x - 1 = x^3 - 1.\]
Let’s now present the examples. They have some very important techniques, so read them carefully before attempting the problems.
In this example we will discuss division with remainder. For polynomials \(f(x)\) and \(g(x)\) with \(\deg(f)\geq \deg(g)\) there always exists polynomials \(q(x)\) and \(r(x)\) such that \[f(x)=q(x)g(x)+r(x)\] and \(\deg(r)<\deg(q)\) or \(r(x)=0\). This should look very much like usual division of numbers, and just like in that case, we call \(f(x)\) the dividend, \(g(x)\) the divisor, \(q(x)\) the quotient, and \(r(x)\) the remainder. If \(r(x)=0\), we say that \(g(x)\) divides \(f(x)\), and we may write \(g(x)\mid f(x)\). Let \(f(x)=x^7-1\) and \(g(x)=x^3+x+1\). Is \(f(x)\) divisible by \(g(x)\)?
In this example we discuss the Factor Theorem. First, let us recall the following concept: if \(P(x)\) is a polynomial, then a number \(z\), is a root of \(P(x)\) if \(P(z)=0\). For example, \(x=1\) is a root of the polynomial \(Q(x)=x-1\). Show:
If \(0\) is a root of \(P(x)\), i.e: \(P(0)=0\), then \(P(x)=xQ(x)\) for some polynomial \(Q(x)\).
Use part 1 to show the Factor Theorem: if \(z\) is a root of \(P(x)\), then \(P(x)=(x-z)K(x)\) for some polynomial \(K(x)\).
In this example we discuss one of Vieta’s formulae. Consider the polynomial \(P(x)=x^2+5x-7\). You can take it as a fact that this polynomial has exactly two distinct roots. What is the sum of its roots? What about their product?
The polynomial \(P(x)=x^3+3x^2-7x+1\) has three distinct roots: \(a,b,\) and \(c\). What is the value of \(a^2+b^2+c^2\)?