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x in (-oo:+oo)
x = (y/4)*(21-z) // - (y/4)*(21-z)
x-((y/4)*(21-z)) = 0
x-1*1/4*y*(21-z) = 0
x-1*1/4*y*(21-z) = 0
x-21/4*y+1/4*y*z = 0
x-21/4*y+1/4*y*z = 0
x-21/4*y+1/4*y*z = 0 // - 1/4*y*z-21/4*y
x = -(1/4*y*z-21/4*y)
x = -(1/4*y*z-21/4*y)
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