Four negatively charged particles each carry a charge qn and form a square of side length d. A positively charged particle carrying a charge qp is at the square's center.
If the arrangement is in equilibrium (the vector sum of the electric forces on each particle is zero), what is the numerical value of the ratio qp/qn?

find the sum of the two horizontal vectors and the sum of the two vertical vectors

isoceles right angled triangle

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