Cancelling out "common terms" on both sides of an equation

You need to be very careful when you do algebra derivations. One of the common mistake is to divide both side by "a common term". Remember you can only do this safely if the "common term" is a constant. However you CAN't do it if it contains a variable.

Example:

x(x-2)=x

You can't cancel out the x on both side and say x=3 is the solution. You must move the x on the right side to the left side.

x(x-2)-x=0

x(x-2-1)=0

The solutions are: x=0 and x=3

The reason why you can't divided both sides by x is that when x is zero, you can't divide anything by zero.

Equally important if not more, is that you CAN'T multiple or divide a "common term" that includes a variable from both side of an inequality. Not only it could be zero, but it could also be negative in which case you would need to flip the sign.

Example:

x^2>x

You CAN'T divided both sides by x and say x>1. What you have to do is to move the right side to the left:

x^2-x>0

x(x-1)>0

Solution would be either both x and x-1 are greater than zero, or both x and x-1 are smaller than zero. So your solution is: x>1 or x<0

Example:

x>1/x

Again you CAN'T multiply both sides by x because you don't know if x is positive or negative. What you have to do is to move the right side to the left:

x-1/x>0

(x^2-1)/x>0

If x>0 then x^2-1>0 =>x>1

If x<0 then x^2-1<0 =>x>-1

Therefore your solution is x>1 or 0>x>-1.

You could also break the original question to two branches from the beginning:

x>1/x

if x>0 then x^2>1 =>x>1

if x<0 then x^2<1 => x>-1

Therefore your solution is x>1 or 0>x>-1.

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