Can someone me with this? given sinx=0.9 , what is cosx ?

We are told that the sine (x) = 0.9 meaning that the sine of angle "x" equals 0.9.
To find the degrees of that angle, we look up the arc sine of 0.9 which equals
64.158 Degrees
So, now we look up the cosine of 64.158 degrees which is 0.43589 and that is what we had to find out. 

Well, if the answer needs to be in a fraction, let's try another approach.
If sine (x) = .9 that means that the opposite side/ hypotenuse = .9 / 1
and that means (by Pythagorean Theorem) the adjacent side^2 = hypotenuse^2 - .9^2
adjacent^2 = 1 -.81
adjacent^2 = .19
so adjacent side = square root of .19
and so the cosine of x = square root of .19 / 1

The decimal expansion would be .27 repeated