The determinant of a matrix 2?×?2 is defined as follows:
A matrix is called degenerate if its determinant is equal to zero.
The norm ||A|| of a matrix A is defined as a maximum of absolute values of its elements.
You are given a matrix 
.
Consider any degenerate matrix B such that norm ||A?-?B|| is
minimum possible. Determine||A?-?B||.
The first line contains two integers a and b (|a|,?|b|?≤?109), the elements of the first row of matrix A.
The second line contains two integers c and d (|c|,?|d|?≤?109) the elements of the second row of matrix A.
Output a single real number, the minimum possible value of ||A?-?B||. Your answer is considered to be correct if its absolute or relative error does not exceed 10?-?9.
1 2 3 4
0.2000000000
1 0 0 1
0.5000000000
In the first sample matrix B is 
In the second sample matrix B is 
这道题可以用二分做，因为要求矩阵最大值的最小值，所以最后A矩阵的每个元素和B矩阵的每个元素差值都等于一个数时zuixiao
codeforces Looksery Cup 2015 H Degenerate Matrix
原文：http://blog.csdn.net/kirito_acmer/article/details/46400069