如何在Golang中使用内置方法？

I am trying this simple code:

var f1 float64 = 23.435
fmt.Println(f1.Acos())

But it gives me the following error:

f1.Acos undefined (type float64 has no field or method Acos)

Can anybody help me in understanding the right way of using the built in methods ?

Acos is a function of the math package, not a built-in method of float64, so you must import it first

import (
    "fmt"
    "math"
)

Then, as per documentation, you pass f1 as an argument to the math.Acos

fmt.Println(math.Acos(f1))