>>49
単純に引数で処理変えたいなら、case文で場合分けすればええんで？
def fn(a,b,c)
case a
when a = 1
puts sum = b + c
puts sum = 2*b
when a = 2
puts sum = b - c
when a = 3
puts sum = b * c
when a = "div"
puts sum = b / c
end
end

fn(1,10,5)
#=>15
#=>20
fn(2,10,5)
#=>5
fn(3,10,5)
#=>50
fn("div",10,5)
#=>2