Question

$\frac{cos18*cos28+cos108*sin208 }{sin18*sin78+sin108*sin168}$

Answer 1

$\frac{cos18*cos28+cos108*sin208 }{sin18*sin78+sin108*sin168}= \frac{cos18*cos28+cos(90+18)*sin(180+28)}{sin18*sin78+sin(90+18)*sin(90+78)}=\frac{cos18*cos28+sin18*sin28}{sin18*sin78+cos18*cos78}=\frac{cos(28-18)}{cos(78-18)}=\frac{cos10}{cos60}=2*cos(10)$