(* 関数定義 2015/10/13 版 *)
myFunc[x_] := x^2

myFunc[10]

myFunc2[x_,y_] := Sqrt[myFunc[x] + myFunc[y]]

myFunc2[3,4]

myFact[0]=1;
myFact[n_] := myFact[n-1]*n

myFact[6]

squareSum[0] = 0
squareSum[n_] := squareSum[n-1] + n^2

squareSum[1]

squareSum[3]