Problem 3: Square Sum (100pts)

写一个代表变量a和b的平方和的算术表达式，来替换掉下面square_sum函数中的下划线。

def square_sum(a, b):
    """Compute the sum of square a and square b.

    Expected result:
    >>> square_sum(3, 4)
    25
    """
    return ______