Question

Plszz answer it..
Trigonometry..
Answer should be zero...​

Answer 1

Answer:

sin(A+B) sin(A–B) = cos²B–cos²A

now,

$sum \: \frac{ {cos}^{2}b - {cos}^{2}a }{ {cos}^{2}a \: {cos}^{2}b } = sum(\frac{1}{ {cos}^{2}a } - \frac{1}{ {cos}^{2} b} ) =$

$sum( {sec}^{2} a - {sec}^{2} b) =$

${sec}^{2} a - {sec}^{2} b + {sec}^{2} b - {sec}^{2} c + {sec}^{2} c - {sec}^{2} a = 0$

Ans