If A,B,C are angles of a …
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Susai Raj 7 years, 6 months ago
We know that A + B +C = 180° (sum of 3 angles of a triangle)
So A = 180-(B+C)
Also cos(180-X) = -cos X .and
cos(X +/- Y) =cosXcosY -/+ sinXsinY.
Now cosA + cos(B-C)= cos{180-(B+C)}+cos(B-C)
=--cos(B+C)+cos(B-C)
=-(cosBcosC-sinBsinC)+cosBcosC+sinB sinC
=-cosBcosC+sinB sinc+cosBcosC+sinB sinC
=2sinBsinC.
2Thank You