Find the principal value of inverse function in radian

:$Cos^{-1}(\frac{1}{√2})$

What is 2/7 of a week (find)?

A speaks truth in 75% cases and B in 80% of the cases. In what percentage of the cases are they likely to contradict each other, narrating the same incident? 

The class mark of class 90-120 is ?

If sinx + siny + sinz = 3. Find the value of cosx + cosy+ cosz.

The manufacturer of a certain item can sell all he can produce at the selling price of Rs 60 each. It costs him Rs 40 in materials and labour to produce each item and he has overhead expenses of Rs 3000 per week in order to operate the plant. The number of units he should produce and sell in order to make a profit of at least Rs 1000 per week, is ?

$N= 9^9$ the N is devisible by how many positive perfect cube.

Find the principal value 

$Cos^{-1} [\frac {-√3}{2}]$

Find the principal value :

$Tan^{-1}[\frac {-1}{√3}]$