Solving simultaneous equation using elimination

Example 1: 

   x-2y = 1 ------(I)

   x+2y = 9 ------(Ii)

All the variables x and y have same coefficients but different signs. We can adopt any sign in order to eliminate one of the variables. We chose addition in order to eliminate y. After getting x, we substituted it equation (I) in order to get y. 

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