Question

see the image and please provide me the reason for that

Answer 1

Hope this will help you.

Answer 2

if you know ,
cosx is a periodic function , e.g value of any trigonometric term { sine , cosine , tangent etc } is repeated again and again with equal interval .

here , cosx = 0
you know ,
cosx = when , x = π/2
but cosx is a periodic function of period ' 2π' . means cosine value repeated after regular interval of 2π .
we also know ,
in 0 to 2π cosx = 0 have two solutions .
e.g x = π/2 and 3π/2
it means in one period there are two value where cosx = 0.
so, period of cosx = 0 is π
hence,
value of x = nπ + π/2 where n is integer .
So, x = (2n + 1)π/2
you can see graph of y = cosx for well understanding.