5(5-3)/2. That it is the formula so that shape would have 5 diagonals. n(n-3)/2 where n is the number of sides
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It is an octagon with 8 sides. Let n be the number of sides = number of vertices. Then each vertex can be connected to n-3 other vertices, giving n(n -3) connections. However, this counts each vertex pair connection twice, so there are: n(n - 3) / 2 diagonals. Thus if there are 20 diagonals: n(n - 3) / 2 = 20 n2 - 3n = 40 n2 - 3n - 40 = 0 (n + 5)(n - 8) = 0 so n = -5 or 8. As a polygon cannot have a negative number of sides, n = 8.
A 9-sided polygon, is named 'Nonagon'. The number of diagonals are '9' From any one node, to the centre of the opposite side. Since there are 9 nodes and 9 lines, then there will be 9 diagonals.
A dodecahedron is a 12 side polyhedron. A regular dodecahedron has regular pentagons (5 sides) for each face.
9x + 5 = 4x + 3 Subtract 4x from both sides: 5x + 5 = 3 Subtract 5 from both sides: 5x = - 2 Divide both sides by 5: x = -2/5 or -0.4
Add 5 to both sides of the equation to get rid of the - 5. -3x - 5 + 5 = -20 + 5 Solve and simplify. -3x = -15 Divide both sides of the equation by -3. x = 5