Book Maths Puzzle Area and Volume

I need help with all this How many distinct diagonals does a hexagon have?
Option
A. 6
B. 9
C. 12
D. 18

Read Solution (Total 3)

9
every vertex has 3 diagonals.
Each diagonal is shared by 2 vertices.
so no. of distinct diagonals = 3*6/2=9
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answer is 9
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Formula = nc2-n
n= no. of side of any polygon
the reason of (-n) is there is n sides in a polygon and we have to find no. of diagonal .Since sides are not a diagonal so we subtract it.
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Book Other Question

A square plate of some size is cut at four corners. Equal squares of the same size are cut and is formed as open box.If this open box carries 128 ml of oil. What is the size of the side of the plate?
I have -3x + 6y = -12, I can workout how to get to x=0,y=-2,

but I'm lost on the x=3,y=-1/2.

-3x + 6y = -12
6y = 3x -12 (take over the 3x to leave 6y)
y =6/3x - 2 (divide by 6 to get to y)
y = 2x - 2 (final answer)

When I enter x = 3, I get y = 4, not y = -1/2.
as
y = 2x -2 = 6 - 2 = 4

Marcus