Use trigonometry.
tan30° = perpendicular / base
Here for angle 30° , x is perpendicular while 10 is the base.
tan30° = 1/√3
1/√3 = x/10
x = 10/✓3
By rationalising
x = 10√3/3
Hope This Helps You!
I think the answer is 84/3
Recall the Maclaurin expansion for cos(x), valid for all real x :

Then replacing x with √5 x (I'm assuming you mean √5 times x, and not √(5x)) gives

The first 3 terms of the series are

and the general n-th term is as shown in the series.
In case you did mean cos(√(5x)), we would instead end up with

which amounts to replacing the x with √x in the expansion of cos(√5 x) :
