Find the positive value of x if the standard de... - JAMB Mathematics 1998 Question
Find the positive value of x if the standard deviation of the numbers 1, x + 1 is 6
A
1
B
2
C
3
D
4
correct option: c
mean (x) = (\frac{1 + x + 1 + 2x + 1}{3})
= (\frac{3x + 3}{3})
= 1 + x
(\begin{array}{c|c} X & (X -X) & (X -X)^2\ \hline 1 & -x & x^2 \ x + 1 & 0 & 0\2x + 1 & x & x^2\ \hline & & 2x^2\end{array})
S.D = (\sqrt{\frac{\sum(x - 7)^2}{\sum f}})
= (\sqrt{(6)}^2)
= (\frac{2x^2}{3})
= 2x2
= 18
x2 = 9
∴ x = (\pm) (\sqrt{9})
= (\pm)3
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