PI = 3.14159265359

def area_of_circle(r):
    if(r<0):
        return None

    return PI*r*r

def sigma(start, end, arg_n):
    if start <0:
        return None
    if start>end:
        return None
    
    _sum=0
    for i in range(start, end):
        _sum+=arg_n(i)
    return _sum

if __name__ == '__main__':
    print('半径为12的圆的面积为:')
    print(area_of_circle(12))
    print('半径为2.5的圆的面积为:')
    print(area_of_circle(2.5))

    
    print('当项x=n时，求当n=1到n=100的和')
    print(sigma(1,101,lambda x: x))
    print('当项x=(n*n+1))时，求当n=1到n=100的和')
    print(sigma(1,101,lambda x: x*x+1 ))