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simultaneous equations
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line line intersection
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x+y=3 & x-y=4
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x+y=3 & y=4-x
2*x+y = 7, x-y/5 = -0.3
2*x+y-3*z= 7, x-y/5+z = 3,  x/3+z=0
linear algebra
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[algebraic manipulation, substitution and elimination of variables,
using addition to eliminate one unknown]
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