A) \[{{b}^{2}}-{{a}^{2}}\]
B) \[{{b}^{2}}-{{c}^{2}}\]
C) \[{{c}^{2}}-{{a}^{2}}\]
D) \[{{a}^{2}}+{{b}^{2}}+{{c}^{2}}\]
Correct Answer: A
Solution :
\[{{b}^{2}}\cos 2A-{{a}^{2}}\cos 2B\]\[={{b}^{2}}(1-2{{\sin }^{2}}A)-{{a}^{2}}(1-2{{\sin }^{2}}B)\] \[={{b}^{2}}-{{a}^{2}}-2({{b}^{2}}{{\sin }^{2}}A-{{a}^{2}}{{\sin }^{2}}B)={{b}^{2}}-{{a}^{2}}\].
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