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If m, n are integers and m + n is odd then the value of ∫0πsin⁡mx.cos⁡nx.dx∫_0^πsin⁡mx.cos⁡nx.dx∫0π​sin⁡mx.cos⁡nx.dx​ is
Question

If m, n are integers and m + n is odd then the value of 0πsinmx.cosnx.dx∫_0^πsin⁡mx.cos⁡nx.dx​ is

A.

0

B.

π/2

C.

π

D.

1

Correct option is A

Given,I=0πsin(mx)cos(nx) dxWe know thatsinAcosB=12[sin(A+B)+sin(AB)]I=0π12[sin(mx+nx)+sin(mxnx)]dxI=120π[sin((m+n)x)+sin((mn)x)]dxI=120πsin((m+n)x) dx+120πsin((mn)x) dxI=12[cos((mn)x)mn]0π+12[cos((m+n)x)m+n]0πI=12[cos((mn)π)mncos((mn)0)mn]+12[cos((m+n)π)m+ncos((m+n)0)m+n]I=0\text{Given,} \\I = \int_0^{\pi} \sin(mx) \cdot \cos(nx) \, dx \\[8pt]\text{We know that} \\\sin A \cdot \cos B = \frac{1}{2} \left[ \sin(A + B) + \sin(A - B) \right] \\[8pt]I = \int_0^{\pi} \frac{1}{2} \left[ \sin(mx + nx) + \sin(mx - nx) \right] dx \\[8pt]I = \frac{1}{2} \int_0^{\pi} \left[ \sin((m + n)x) + \sin((m - n)x) \right] dx \\[8pt]I = \frac{1}{2} \int_0^{\pi} \sin((m + n)x) \, dx + \frac{1}{2} \int_0^{\pi} \sin((m - n)x) \, dx \\[8pt]I = -\frac{1}{2} \left[ \frac{\cos((m - n)x)}{m - n} \right]_0^{\pi} + \frac{1}{2} \left[ \frac{\cos((m + n)x)}{m + n} \right]_0^{\pi} \\[8pt]I = -\frac{1}{2} \left[ \frac{\cos((m - n)\pi)}{m - n} - \frac{\cos((m - n) \cdot 0)}{m - n} \right] + \frac{1}{2} \left[ \frac{\cos((m + n)\pi)}{m + n} - \frac{\cos((m + n) \cdot 0)}{m + n} \right] \\[8pt]I = 0​​

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