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Ask iit jee aieee pet cbse icse state board community Community Discussion Question: mean value th probs
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abhijeet_0201 (756)

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1)if a+b+c=0 then the equation 3ax^2 +2bx+c has in the interval(0,1)


a at least one root       b at the most one root      c no root          d none of these


2)if 27a+9b+3c+d=0   then the equation  4ax^3+3bx^2+2cx+d=0 has at least one root lying b/w


a  0 and 1        b 1and3        c 0and 3                    dnone of these


 


plz ans as soon as possible!!!!


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computer001 (1847)

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1) a
i think...
if f(0)>0 v get f(2/3)<0 n vice versa
if f(1/3)>0 then f(1)<0 n vice versa

so between 0 and 2/3 therer is a root similarly there is a root between 1/3 and 1...these 2 roots may be same or diff...so there is atleast 1 root

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abhijeet_0201 (756)

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ur ans looks correct but then whats the use of equation a+b+c=0???
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computer001 (1847)

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only using that i get:


if f(0)>0 v get f(2/3)<0 n vice versa

if f(1/3)>0 then f(1)<0 n vice versa


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abhijeet_0201 (756)

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cud u plz be aliitle elaborate???
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computer001 (1847)

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2)


consider the eqn:


g(x)=ax^4 + bx^3 + cx^2 +dx +e


here g'(x)=eqn u mentioned in the question


g(0)=e


g(3) also =e


so g(0)=g(3)


hence from rolles theorem there exists values of x (0,3) such tht g'(x)=0


these values may be 1 or more


hence the answer is atleast 1 root in (0,3)


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computer001 (1847)

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1)consider g(x)=ax^3 + bx^2 + cx +d


g(0)=d


g(1)=a+b+c+d=d(as a+b+c=0)


g(0)=g(1)...from rolles theorem there exist atleast 1 value in (0,1) such tht g'(x)=0


 


note tht g'(x) = the eqn u have given


hence ans is ---atleast 1 root


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abhijeet_0201 (756)

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thks!!!
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