Let $P_2$ be the space of polynomials of degree at most 2. An inner product on this space is defined by the formula: $(p, q) = \frac{1}{8} \int_{-1}^{1} p(x) q(x) dx$ We need to find a polynomial $h(x) = h_0 + h_1 x + h_2 x^2 \in P_2$ such that $(p, h) = p(2)$ for every $p(x) \in P_2$. We need to find the coefficients $h_0, h_1, h_2$.

AnalysisInner ProductPolynomialsIntegrationLinear AlgebraOrthogonal Polynomials
2025/5/14

1. Problem Description

Let P2P_2 be the space of polynomials of degree at most

2. An inner product on this space is defined by the formula:

(p,q)=1811p(x)q(x)dx(p, q) = \frac{1}{8} \int_{-1}^{1} p(x) q(x) dx
We need to find a polynomial h(x)=h0+h1x+h2x2P2h(x) = h_0 + h_1 x + h_2 x^2 \in P_2 such that (p,h)=p(2)(p, h) = p(2) for every p(x)P2p(x) \in P_2. We need to find the coefficients h0,h1,h2h_0, h_1, h_2.

2. Solution Steps

Since (p,h)=p(2)(p, h) = p(2) for every pP2p \in P_2, we can choose some specific polynomials p(x)p(x) to obtain equations for h0,h1,h2h_0, h_1, h_2. We'll choose p(x)=1,p(x)=x,p(x)=x2p(x) = 1, p(x) = x, p(x) = x^2.
Case 1: p(x)=1p(x) = 1.
Then p(2)=1p(2) = 1. Also, (1,h)=1811(h0+h1x+h2x2)dx=18[h0x+h1x22+h2x33]11=18(2h0+2h23)=h04+h212(1, h) = \frac{1}{8} \int_{-1}^{1} (h_0 + h_1 x + h_2 x^2) dx = \frac{1}{8} [h_0 x + \frac{h_1 x^2}{2} + \frac{h_2 x^3}{3}]_{-1}^{1} = \frac{1}{8} (2h_0 + \frac{2h_2}{3}) = \frac{h_0}{4} + \frac{h_2}{12}.
So, h04+h212=1\frac{h_0}{4} + \frac{h_2}{12} = 1. This implies 3h0+h2=123h_0 + h_2 = 12.
Case 2: p(x)=xp(x) = x.
Then p(2)=2p(2) = 2. Also, (x,h)=1811x(h0+h1x+h2x2)dx=1811(h0x+h1x2+h2x3)dx=18[h0x22+h1x33+h2x44]11=18(2h13)=h112(x, h) = \frac{1}{8} \int_{-1}^{1} x(h_0 + h_1 x + h_2 x^2) dx = \frac{1}{8} \int_{-1}^{1} (h_0 x + h_1 x^2 + h_2 x^3) dx = \frac{1}{8} [\frac{h_0 x^2}{2} + \frac{h_1 x^3}{3} + \frac{h_2 x^4}{4}]_{-1}^{1} = \frac{1}{8} (\frac{2h_1}{3}) = \frac{h_1}{12}.
So, h112=2\frac{h_1}{12} = 2. This implies h1=24h_1 = 24.
Case 3: p(x)=x2p(x) = x^2.
Then p(2)=4p(2) = 4. Also, (x2,h)=1811x2(h0+h1x+h2x2)dx=1811(h0x2+h1x3+h2x4)dx=18[h0x33+h1x44+h2x55]11=18(2h03+2h25)=h012+h220(x^2, h) = \frac{1}{8} \int_{-1}^{1} x^2 (h_0 + h_1 x + h_2 x^2) dx = \frac{1}{8} \int_{-1}^{1} (h_0 x^2 + h_1 x^3 + h_2 x^4) dx = \frac{1}{8} [\frac{h_0 x^3}{3} + \frac{h_1 x^4}{4} + \frac{h_2 x^5}{5}]_{-1}^{1} = \frac{1}{8} (\frac{2h_0}{3} + \frac{2h_2}{5}) = \frac{h_0}{12} + \frac{h_2}{20}.
So, h012+h220=4\frac{h_0}{12} + \frac{h_2}{20} = 4. This implies 5h0+3h2=2405h_0 + 3h_2 = 240.
Now we have a system of equations:
3h0+h2=123h_0 + h_2 = 12
h1=24h_1 = 24
5h0+3h2=2405h_0 + 3h_2 = 240
Multiply the first equation by 3: 9h0+3h2=369h_0 + 3h_2 = 36.
Subtract this from the third equation: (5h0+3h2)(9h0+3h2)=24036(5h_0 + 3h_2) - (9h_0 + 3h_2) = 240 - 36, so 4h0=204-4h_0 = 204.
Thus, h0=51h_0 = -51.
Substitute h0=51h_0 = -51 into 3h0+h2=123h_0 + h_2 = 12: 3(51)+h2=123(-51) + h_2 = 12, so 153+h2=12-153 + h_2 = 12.
Thus, h2=165h_2 = 165.
Therefore, h(x)=51+24x+165x2h(x) = -51 + 24x + 165x^2.

3. Final Answer

h0=51h_0 = -51, h1=24h_1 = 24, h2=165h_2 = 165
h(x)=51+24x+165x2h(x) = -51 + 24x + 165x^2

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