Trigonometry
Determine the exact value of the expression without using a calculator.
cos(tan−1940−tan−12021)\cos\left(\tan^{-1}\frac{9}{40}-\tan^{-1}\frac{20}{21}\right)cos(tan−1409−tan−12120)
−6111189-\frac{611}{1189}−1189611
7891160\frac{789}{1160}1160789
6601189\frac{660}{1189}1189660
10201189\frac{1020}{1189}11891020