If x=cy+bz,y=cx+az,z=bx+ay the value of a2+b2−1 is
A
abc
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B
-abc
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C
2abc
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D
-2abc
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Solution
The correct option is D -2abc x=1y+bzy=cx+azz=bx+ayx=cy+b(bx+ay)x=cy+b2x+aby(1−b2)x=(ab+c)yxy=ab+c1−b2...(1)y=cx+az=cx+a(bx+ay)y=cx+abx+a2y(1−a2)y=x(c+ab)xy=1−a2c+abyx=c+ab1−a2...(2) Multiplying 1 and 2 1=(ab+c)2(1−a2)(1−b2)(1−a2)(1−b2)=a2b2+c2+7abc1−a2−b2+/a2b2=/a2b2+c2+2abca2+b2+c2+2abc−1=0a2+b2+c2−1=−2abc