If A + B + C = pi/2, find the value of tan(A)*tan(B) + tan(B)*tan(C) + tan(C)*tan(A).
1
0
-1
1/2
Correct answer: 1
Solution
From A+B = pi/2 - C, tan(A+B) = cot(C); cross-multiplying gives tanA tanC + tanB tanC = 1 - tanA tanB, which rearranges to the required sum equal to 1.