© examsiri.com
Question : 107 of 160
Marks:
+1,
-0
Solution:
Given that,
A+B=CNow,
cos2A+cos2B+cos2C‌‌=‌+‌+cos2C‌‌=1+‌(cos‌2‌A+cos‌2‌B)+cos2C‌‌=1+‌[cos(A+B)‌cos(A−B)]+cos2C‌‌=1+cos‌C[cos(A−B)+cos(A+B)]‌‌=1+2‌cos‌C‌cos‌B‌cos‌A‌⇒cos2A+cos2B+cos2C−2‌cos‌A‌cos‌B‌cos‌C=1
© examsiri.com
Go to Question: