Math Problem Statement

Use induction to prove that sinx+sin3x+sin5x+⋯+sin(2n−1)x=1−cos2nx/2sinx for all n≥1 .

Solution

To prove the statement using mathematical induction:

sinx+sin3x+sin5x++sin(2n1)x=1cos(2nx)2sinx,for alln1,\sin x + \sin 3x + \sin 5x + \dots + \sin(2n-1)x = \frac{1 - \cos(2nx)}{2 \sin x}, \quad \text{for all} \, n \geq 1, we follow the standard steps of induction.

Step 1: Base Case (n = 1)

For n=1n = 1, the left-hand side (LHS) of the equation is: sinx.\sin x. The right-hand side (RHS) for n=1n = 1 is: 1cos(21x)2sinx=1cos(2x)2sinx.\frac{1 - \cos(2 \cdot 1 \cdot x)}{2 \sin x} = \frac{1 - \cos(2x)}{2 \sin x}. Now, using the identity: 1cos(2x)=2sin2x,1 - \cos(2x) = 2 \sin^2 x, we substitute into the RHS: 1cos(2x)2sinx=2sin2x2sinx=sinx.\frac{1 - \cos(2x)}{2 \sin x} = \frac{2 \sin^2 x}{2 \sin x} = \sin x. Thus, for n=1n = 1, both sides are equal, confirming the base case.

Step 2: Inductive Hypothesis

Assume that the formula holds for some n=kn = k, i.e., sinx+sin3x+sin5x++sin(2k1)x=1cos(2kx)2sinx.\sin x + \sin 3x + \sin 5x + \dots + \sin(2k-1)x = \frac{1 - \cos(2kx)}{2 \sin x}. We need to prove that the formula also holds for n=k+1n = k + 1.

Step 3: Inductive Step

For n=k+1n = k+1, the LHS becomes: sinx+sin3x+sin5x++sin(2k1)x+sin(2(k+1)1)x=sinx+sin3x++sin(2k1)x+sin(2k+1)x.\sin x + \sin 3x + \sin 5x + \dots + \sin(2k-1)x + \sin(2(k+1)-1)x = \sin x + \sin 3x + \dots + \sin(2k-1)x + \sin(2k+1)x. By the inductive hypothesis, we can substitute for the sum of the first kk terms: 1cos(2kx)2sinx+sin(2k+1)x.\frac{1 - \cos(2kx)}{2 \sin x} + \sin(2k+1)x.

We now combine these terms. Using the sum identity for sine: sinA+sinB=2sin(A+B2)cos(AB2),\sin A + \sin B = 2 \sin\left(\frac{A + B}{2}\right) \cos\left(\frac{A - B}{2}\right), let A=(2k+1)xA = (2k+1)x and B=(2kx)B = (2kx), but this step will involve exploring direct term transformations as suitable for your problem

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Math Problem Analysis

Mathematical Concepts

Mathematical Induction
Trigonometric Series
Sum of Sines
Trigonometric Identities

Formulas

sin A + sin B = 2 sin((A + B) / 2) cos((A - B) / 2)
1 - cos(2x) = 2sin^2(x)

Theorems

Principle of Mathematical Induction
Sum of Sine Functions Identity

Suitable Grade Level

Undergraduate Mathematics or Advanced High School