A) \[\sigma =\sqrt{\left( \frac{\sum \,fd}{\sum \,f} \right)-\frac{\sum \,f{{d}^{2}}}{\sum \,f}}\]
B) \[\sigma =\sqrt{\frac{\sum \,f{{d}^{2}}}{\sum \,f}-{{\left( \frac{\sum \,f{{d}^{2}}}{\sum \,f} \right)}^{2}}}\]
C) \[\sigma =\sqrt{{{\left( \frac{\sum \,fd}{\sum \,f} \right)}^{2}}-\frac{\sum \,f{{d}^{2}}}{\sum \,f}}\]
D) \[\sigma =\sqrt{\frac{\sum \,f{{d}^{2}}}{\sum \,f}-{{\left( \frac{\sum \,fd}{\sum \,f} \right)}^{2}}}\]
Correct Answer: D
Solution :
It is obvious.
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