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En una variable X, s'han observat els següents valors presentats com dades agrupades
| Xi | ni |
| 4 | 1 |
| 6 | 3 |
| 8 | 5 |
| 9 | 7 |
| 11 | 2 |
| 12 | 1 |
Calcula el sumatori de quadrats de les puntuacions.