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Question

If x−y+z=5 and x2+y2+z2=49, then the value of zx−xy−yz is

A
12
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B
12
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C
37
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D
37
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Solution

The correct option is D 12
Given,
xy+z=5 and x2+y2+z2=49
We know,
(a+b+c)2=a2+b2+c2+2(ab+bc+ca)
Here,
a=x
b=y
c=z
Thus,
(xy+z)2=x2+y2+z2+2(xyyz+zx)
52=49+2(xyyz+zx)
2(xyyz+zx)=4925
(xyyz+zx)=242
(xyyz+zx)=12

Hence, option B.

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