Use an addition or subtraction formula to write the expression as a trigonometric function of one angle:

cos⁡3π7cos⁡2π21+sin⁡3π7sin⁡2π21=\displaystyle {\cos{{\frac{{{3}\pi}}{{{7}}}}}}{\cos{{\frac{{{2}\pi}}{{{21}}}}}}+{\sin{{\frac{{{3}\pi}}{{{7}}}}}}{\sin{{\frac{{{2}\pi}}{{{21}}}}}}=  
The exact value of this function is   .

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