设 x = a+d = b+c,显然,x 不是奇数,
否则,b,c 必然为一个奇数一个偶数,它们的乘积是偶数;
同理,a,d 的乘积也是偶数,那么 bc - ad 是偶数,与 bc - ad = 69 矛盾
因此,我们可以设 2n = a + d = b + c,有 l > k 使得:
b = n - k,c = n + k
a = n - l,d = n + l
bc = n^2 - k^2
ad = n^2 - l^2
bc - ad = l^2 - k^2 = (l-k)(l+k) = 69 = 3*23
有两种情况满足上述条件,1)
l - k = 1
l + k = 69
此时 l = 35,k = 34
2)
l - k = 3
l + k = 23
此时,l = 13,k = 10
对于 a),由题目条件 0