A) \[{{\cos }^{2}}A-1\]
B) \[2{{\cos }^{2}}A-1\]
C) \[2{{\cos }^{2}}A+1\]
D) \[{{\cos }^{2}}A+1\]
Correct Answer: B
Solution :
\[x={{\cos }^{4}}A-{{\sin }^{4}}A\] \[=(co{{s}^{2}}A+si{{n}^{2}}A)(co{{s}^{2}}A-si{{n}^{2}}A)\] \[={{\cos }^{2}}A-{{\sin }^{2}}A={{\cos }^{2}}A-(1-{{\cos }^{2}}A)\] \[=2{{\cos }^{2}}A-1\]
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