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If P = 7 + 4√3 and PQ = 1, then what is the value of $$\frac{1}{{{P^2}}} + \frac{1}{{{Q^2}}}?$$
Answer & Solution
Correct Answer:
Option
B
$$\eqalign{
& P = 7 + 4\sqrt 3 \cr
& PQ = 1,\,Q = \frac{1}{P} \cr
& Q = \frac{1}{{7 + 4\sqrt 3 }} = 7 - 4\sqrt 3 \cr
& P + Q \cr
& = 7 + 4\sqrt 3 + 7 - 4\sqrt 3 \cr
& = 14 \cr
& \Rightarrow \frac{1}{{{P^2}}} + \frac{1}{{{Q^2}}} \cr
& = \frac{{{P^2} + {Q^2}}}{{{{\left( {PQ} \right)}^2}}} \cr
& = \frac{{{{\left( {P + Q} \right)}^2} - 2PQ}}{{{{\left( {PQ} \right)}^2}}} \cr
& = {\left( {14} \right)^2} - 2 \cr
& = 196 - 2 \cr
& = 194 \cr} $$
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