Q.

 Let the function f:R be defined by f(x)=x3x2+(x1)sinx and let g:RR be an 

 arbitrary function. Let fg : be the product function defined by (fg)(x)=f(x)g(x) . Then 

 which of the following statements is/are TRUE ? 

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a

 If fg is differentiable at x=1 , then g is continuous at x=1

b

 If fg is differentiable at x=1 , then g is differentiable at x=1

c

 If g is differentiable at x=1 , then fg is differentiable at x=1

d

 If g is continuous at x=1 , then fg is differentiable at x=1

answer is A, C.

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Detailed Solution

f:RR f(x)=x2+sinx(x1) f1+=f1=f(1)=0fg(x):f(x)g(x) fg:RR let fg(x)=h(x)=f(x)g(x) h:RR option (c): h(x)=f(x)g(x)+f(x)g(x) 

                                                             h(1)=f(1)g(1)+0                                                         as f(1)=0,g(x) exists 

 if g(x) is differentiable then h(x) is also differentiable (true)  option (A) : If g(x) is continuous at x=1 then g1+=g1=g1                  h1+=limh0+h(1+h)h(1)h

h1+=limh0+f(1+h)g(1+h)0h=f(1)g(1)h1=limh0+f(1h)g(1h)0h=f(1)g(1) So h(x)=f(x)g(x) is differentiable  at x=1  (True) 

 option (B) (D): h1+=limh0+h(1+h)h(1)h

h1+=limh0+f(1+h)g(1+h)h=f(1)g1+h1=limh0+f(1h)g(1h)h=f(1)g1g1+=g1

so we cannot comment on the continuity and differentiability of the function gx

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