Why is big int api in Go so strange?

I'm trying to understand the design decisions behind the big int api.

For example to add two big ints you have to:

a := big.NewInt(10)
b := big.NewInt(20)
c := big.NewInt(0)
d := c.Add(a,b)

where d is the same as c at the end. The initial zero does not matter a bit.

Why not just:

a := big.NewInt(10)
b := big.NewInt(20)
c := big.Add(a,b)

Or better yet:

a := big.NewInt(10)
b := big.NewInt(20)
c := a.Add(b)

Is there any reason they chose to do it this way? I find it little confusing and have to look it up whenever I use it.