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Question

If in ABC,A=π4 and tanBtanC=p, then the possible set of value(s) of p is/are

A
(,0)
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B
((2+1)2,)
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C
[(2+1)2,)
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D
(,0]
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Solution

The correct options are
A (,0)
C [(2+1)2,)
Given: A+B+C=π,A=π4
B+C=3π43π4>B,C>0
Also,
tanBtanC=psinBsinCcosBcosC=p1cosBcosCsinBsinCcosBcosC+sinBsinC=1p1+pcos(B+C)cos(BC)=1+p1p12cos(BC)=1+p1pcos(BC)=1+p2(p1)

Now, we know that
3π4>BC01cos(BC)>1211+p2(p1)>1221+pp1>1
Solving both inequality, we get
1+pp1>11+pp1+1>02pp1>0p(,0)(1,)(1)

21+pp11+pp120p(12)+(1+2)p10p+(2+1)2p10p(2+1)2p10p(,0)[(2+1)2,)(2)

From equation (1) and (2), we get
p(,0)[(2+1)2,)

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