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Find three numbers in G.P. whose sum is 13 and the sum of whose squaresis 91. |
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Answer» Let the required numbers be `a/r, a` and `ar`. Then, `a/r+a+ar=13` ...(i) and `a^(2)/r^(2)+a^(2)+a^(2)r^(2)=91`...(ii) On squaring both sides of (i), we get `(a/r+a+ar)^(2)=169` `rArr (a^(2)/r^(2)+a^(2)+a^(2)r^(2))+2 (a^(2)/r+a^(2)+a^(2)r)=169` [using (ii)] `rArr 91+26a=169` [using (i)] `rArr 26a=78 rArr a=3`. Putting `a=3` in (i), we get `3/r+3+3r=13 rArr 3/r +3r=10 rArr 3r^(2)-10r+3=0` `rArr (r-3)(3r-1)=0rArr r=3 or r=1/3`. Hence, the required numbers are 1, 3, 9 or 9, 3, 1. |
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