The Sigma Insight: Fundamental Theorem & Properties of Definite Integrals
Solution Diagram
Analyzing the Polynomial Constraints
We are tasked with finding the number of polynomials f(x)=ax2+bx+c with non-negative integer coefficients such that f(0)=0 and ∫01f(x)dx=1.
The condition f(0)=0 immediately implies that c=0. Thus, our polynomial simplifies to the form f(x)=ax2+bx.
Integrating this expression from 0 to 1, we obtain:
∫01(ax2+bx)dx=[3ax3+2bx2]01=3a+2b=1
Multiplying the entire equation by 6, we arrive at the linear Diophantine equation:
2a+3b=6
Since a and b must be non-negative integers, we test the possible values for b:
1. If b=0, then 2a=6, which gives a=3.
2. If b=2, then 3b=6, which gives 2a=0, so a=0.
Any other value for b results in a being either non-integer or negative. Therefore, there are exactly 2 such polynomials.
The Dance of Trigonometric Maxima
We examine the function f(x)=sin(x2)+cos(x2) on the interval [−13,13]. This expression follows the form Asinθ+Bcosθ, which can be rewritten as:
f(x)=2sin(x2+4π)
The maximum value of this function is 2, which occurs when the argument of the sine function is 2nπ+2π. Setting the argument accordingly:
x2+4π=2nπ+2π⇒x2=2nπ+4π
Given the interval [−13,13], we have x2∈[0,13]. We test values of n:
1. For n=0, x2=4π≈0.78, which yields two points: x=±4π.
2. For n=1, x2=2π+4π=49π≈7.06, which yields two more points: x=±49π.
3. For n=2, x2=4π+4π≈13.35, which is outside the allowed range.
In total, there are 4 points where the function reaches its maximum.
The Elegance of King's Property
We evaluate the integral I=∫−221+ex3x2dx. We apply King's property, which states that ∫−aaf(x)dx=∫−aaf(−x)dx.
Replacing x with −x, we get:
I=∫−221+e−x3x2dx=∫−22ex+13x2exdx
Adding the two versions of I together:
2I=∫−221+ex3x2(1+ex)dx=∫−223x2dx
Evaluating this integral:
2I=[x3]−22=8−(−8)=16
Thus, the final value is I=8.
The Power of Symmetry
Finally, we consider the integral of g(x)=cos(2x)log(1−x1+x). We check the parity of the function by evaluating g(−x):