Question

find the value of cos(1°+2°+3°+....9°)

Answer 1

first find
sum of (1+2+3+....+9)=9 (9+1)/2=45 degree
here we use sum of n natural number
=n (n+1)/2

now
cos (1+2+3....+9)=cos45=1/root2

Answer 2

sum of (1+2+3+....+9)=9 (9+1)/2=45 degree 
 sum of n natural number
=n (n+1)/2


cos (1+2+3....+9)=cos45=1/root2