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If `x_1,x_2 …,x_(20)` are in H.P and `x_1,2,x_(20)` are in G.P then `Sigma_(r=1)^(19)x_rr_(x+1)`A. 76B. 80C. 84D. none of these |
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Answer» Correct Answer - A Clearly, `1/x_(1),1/x_(2),…1/x_(20)` will be in A.P. Hence, `1/x_(2)-1/x_(1)=1/x_(3)-1/x_(2)=..=1/x_(r+1)-1/x_(r)=..=lamda` (say) `rArr(x_(r)-x_(r+1))/(x_(r)x_(r+1))=lamda` or `x_(r)x_(r+1)=-1/lamda(x_(r+1)-x_(r ))` `rArrsum_(r=1)^(19)x_(r )x_(r+1)=-1/lamdasum_(r=1)^(19)(x_(r+1)-x_(r ))=-1/lamda(x_(20)-x_(1))` Now, `1/x_(20)=1/x_(1)+19lamda` or `(x_(1)-x_(20))/(x_(1)x_(20))=19lamda` `rArrsum_(r=1)^(19)x_(r )x_(r+1)=19x_(1)x_(20)=19xx4=76` `(becausex_(1),2,x_(20)` are in G.P., then `x_(1)x_(20)=4)` |
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