The correct options are
A (−∞,0)
C [(√2+1)2,∞)
Given: A+B+C=π,∠A=π4
⇒B+C=3π4⇒3π4>B,C>0
Also,
tanBtanC=p⇒sinBsinCcosBcosC=p1⇒cosBcosC−sinBsinCcosBcosC+sinBsinC=1−p1+p⇒cos(B+C)cos(B−C)=1+p1−p⇒−1√2cos(B−C)=1+p1−p⇒cos(B−C)=1+p√2(p−1)
Now, we know that
3π4>B−C≥0⇒1≥cos(B−C)>−1√2⇒1≥1+p√2(p−1)>−1√2⇒√2≥1+pp−1>−1
Solving both inequality, we get
1+pp−1>−1⇒1+pp−1+1>0⇒2pp−1>0⇒p∈(−∞,0)∪(1,∞)⋯(1)
√2≥1+pp−1⇒1+pp−1−√2≤0⇒p(1−√2)+(1+√2)p−1≤0⇒−p+(√2+1)2p−1≤0⇒p−(√2+1)2p−1≥0⇒p∈(−∞,0)∪[(√2+1)2,∞)⋯(2)
From equation (1) and (2), we get
p∈(−∞,0)∪[(√2+1)2,∞)