Practice question

**Independent action of charges** allows total force or field as vector sum. Geometry dictates distances to evaluation point, and resultant follows Σ k q_i/r_i², explaining zero field at symmetric centres for equal charges. Let x be distance from +15 μC , then 0.75 - x from -5 μC . (15 × 10⁻⁶/x²) = (5 × 10⁻⁶/(0.75 - x)²) , 15 (0.75 - x)² = 5 x² . 3 (0.5625 - 1.5 x + x²) = x² , 1.6875 - 4.5 x + 3 x² = x² . 2 x² - 4.5 x + 1.6875 = 0 , x = (4.5 ± √(20.25 - 13.5)/4) =