Solution
Core Logic
For a polynomial P(x) = x³ + ax² + bx + c to be divisible by x² + 2, we can perform polynomial long division or use synthetic substitution (roots of x²+2=0). Alternatively, factorize P(x): Since the degree is 3 and the leading coefficient is 1, we must have:
x³ + ax² + bx + c = (x² + 2)(x + k)where k is a constant.
Step 1: Equating Coefficients
Expand the right side:
(x² + 2)(x + k) = x³ + kx² + 2x + 2kComparing coefficients with x³ + ax² + bx + c: x² coefficient: a = k x coefficient: b = 2 Constant term: c = 2k
Step 2: Apply Constraints
From the coefficient matching, we have a = (c)/(2) and b = 2. Since a, b, c in N and a, b, c ≤ 20: b = 2 (this is fixed, always valid). c must be an even natural number such that c ≤ 20. The possible values for c are 2, 4, 6, 8, 10, 12, 14, 16, 18, 20. For each such c, a is uniquely determined as a = c/2 and a ≤ 10 (which easily satisfies a ≤ 20).
Step 3: Final Count
The number of valid (a, b, c) tuples corresponds to the number of valid c values. There are exactly 10 values for c. Thus, the number of polynomials in S is 10.
Chapter Mix
Class 10 Mathematics: Polynomials Class 11 Mathematics: Complex Numbers and Quadratic Equations