1 2 3 4 5 what symbol to fill in the middle = 20

1+2-3+4x5
1+2+3x4+5
1+2x3x4-5
1+2x(3+4)+5

1 × 3 + 1 = 4 = 2, 2 × 4 + 1 = 9 = 3, 3 × 5 + 1 = 16 = 4

N × (n + 2) + 1 = (n + 1) square

How to calculate the square of a (A-1) = 9

A (A-1) = plus or minus 3
A2-a-3 = 0 or a2-a + 3 = 0
If a2-a-3 = 0
A1 = - 1 / 2 + radical 13 / 2
A2 = - 1 / 2 - radical 13 / 2
If a2-a + 3 = 0
This equation has no solution
So the solution of the original equation is
A1 = - 1 / 2 + radical 13 / 2
A2 = - 1 / 2 - radical 13 / 2

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