We know that WORK=FORCE MULTIPLY BY DISPLACEMENT COS TETA ???what do you mean by cos teta ??? I did not understand what is cos teta and sin teta???

The answer is Cos teta because,
There are two types of vector products, namely,
1. Scalar or dot product
2. Vector  or cross product
Here,

W = F . S (dot product))
where  W is work
           F is force  and
           S is displacement 
 W = F . S,  " . "  dot here refers to Dot product of a vector or scalar product of a vector
On removing the dot we get cos teta in scalar or dot product
Example, A . B = ABCos teta

In cross product of vectors
For example,
A x B gives AB Sin teta

A x B = AB Sin teta