```Question 536171
{{{drawing(400,400,-7,7,-7,7, graph(400,400,-7,7,-7,7),

circle(-1,1,.1), circle(0,-3,.1),circle(3,5,.1),circle(4,1,.1),
red(line(-1,1,0,-3),line(4,1,3,5),line(4,1,0,-3),line(-1,1,3,5)),
green(line(-1,1,4,1),line(0,-3,3,5)), locate(-3.5+.7,1+.2,"(-1,1)"),
locate(0+.2,-3+.3,"(0,-3)"), locate(4+.2,1+.3,"(4,1)"), locate(3+.2,5+.3,"(3,5)")

)}}}
<pre>
We just need to show that the midpoint of each diagonal is the same point.

We find the midpoint of the diagonal connecting (-1,1) and (4,1)

Midpoint = {{{(matrix(1,3,      (x[1]+x[2])/2,   ",", (y[1]+y[2])/2))}}}

Midpoint = {{{(matrix(1,3,      (-1+4)/2,   ",", (1+1)/2))}}}

Midpoint = {{{(matrix(1,3,      3/2,   ",", 2/2))}}}

Midpoint = {{{(matrix(1,3,      3/2,   ",", 1))}}}

We find the midpoint of the diagonal connecting (0,-3) and (3,5)

Midpoint = {{{(matrix(1,3,      (x[1]+x[2])/2,   ",", (y[1]+y[2])/2))}}}

Midpoint = {{{(matrix(1,3,      (0+3)/2,   ",", (-3+5)/2))}}}

Midpoint = {{{(matrix(1,3,      3/2,   ",", 2/2))}}}

Midpoint = {{{(matrix(1,3,      3/2,   ",", 1))}}}

Since the two diagonals have the same midpoint, they bisect each other.

Edwin</pre>```