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identify which are functions

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Which of  the following are not functions  (from R to R )and why

a) f(x)= sqrt(x)

b)f(x) = sqrt(x2 +1)

c)f(x)=   + -  sqrt(x2+1)

d) f(x)=+- x

e) f(x)=1/(n2-4)

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a) not a function as -ve x are not allowed

b) a function as all x are defined and maps to diff location

c)    d)   are not functions as same x values are mapped to diff co domain

e)not a function as x=2 is not defined
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