If A − B = π/4, then (1 + tan A) (1 − tan B) is equal to

Question:

If A − B = π/4, then (1 + tan A) (1 − tan B) is equal to

(a) 2

(b) 1

(c) 0

(d) 3

Solution:

(a) 2

$\tan (A-B)=\tan \frac{\pi}{4}$

$\Rightarrow \frac{\tan A-\tan B}{1+\tan A \tan B}=1$

 

$\Rightarrow \tan A-\tan B=1+\tan A \tan B \quad \ldots(1)$

Now,

$(1+\tan A)(1-\tan B)=1+\tan A-\tan B-\tan A \tan B$

$=1+1+\tan A \tan B-\tan A \tan B \quad($ Using eq $(1))$

 

$=2$

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