The problem asks us to first show a reduction formula for the integral of cosnx using integration by parts, and then use it to evaluate a definite integral.
Part 1: Show that nIn=sinxcosn−1x+(n−1)In−2 for In=∫cosnxdx.
Step 1: Set up the integral for integration by parts.
Let In=∫cosnxdx=∫cosn−1x⋅cosxdx.
We use integration by parts formula: ∫udv=uv−∫vdu.
Let u=cosn−1x and dv=cosxdx.
Step 2: Find du and v.
Differentiating u: du=(n−1)cosn−2x(−sinx)dx=−(n−1)cosn−2xsinxdx.
Integrating dv: v=∫cosxdx=sinx.
Step 3: Apply the integration by parts formula.
In=cosn−1xsinx−∫sinx⋅(−(n−1)cosn−2xsinx)dx
In=cosn−1xsinx+(n−1)∫sin2xcosn−2xdx
Step 4: Use the identity sin2x=1−cos2x.
In=cosn−1xsinx+(n−1)∫(1−cos2x)cosn−2xdx
In=cosn−1xsinx+(n−1)∫(cosn−2x−cosnx)dx
In=cosn−1xsinx+(n−1)∫cosn−2xdx−(n−1)∫cosnxdx
Step 5: Substitute In−2 and In back into the equation.
In=cosn−1xsinx+(n−1)In−2−(n−1)In
Step 6: Rearrange the equation to solve for In.
In+(n−1)In=cosn−1xsinx+(n−1)In−2
In(1+n−1)=cosn−1xsinx+(n−1)In−2
nIn=cosn−1xsinx+(n−1)In−2
This proves the reduction formula.
Part 2: Hence evaluate ∫02πcos5xdx.
Step 1: Apply the reduction formula to the definite integral.
Let Jn=∫02πcosnxdx.
Using the reduction formula from Part 1:
nJn=[sinxcosn−1x]02π+(n−1)Jn−2
Step 2: Evaluate the boundary term.
For n>1:
[sinxcosn−1x]02π=sin(2π)cosn−1(2π)−sin(0)cosn−1(0)
=(1)⋅(0)−(0)⋅(1)=0
So, for n>1, the reduction formula for definite integrals from 0 to 2π simplifies to:
nJn=(n−1)Jn−2⟹Jn=nn−1Jn−2
Step 3: Use the reduction formula to evaluate J5.
J5=55−1J5−2=54J3
J3=33−1J3−2=32J1
Step 4: Calculate J1.
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