DragginMath: Substitution

Sometimes you must substitute one value for another in an expression. DragginMath can do this.

When substituting, you must have at least two expressions on the screen, and at least one of them must be an equation.

Enter 5x↑2+6x–7; x=3. From the equation, drag x into either Blue Mode or Green Mode, then onto – in the other expression. The result is 5∗3↑2+6∗3–7; x=3. All occurrences of x in the expression were replaced with 3. Since we are just playing with this, now use the other side of the equation to replace all occurrences of 3 with x.

The old value you are replacing does not have to be simple.

The new value you are substituting does not have to be simple.

For example, enter 5x↑2+6x–7; x↑2=a+b. Drag ↑ from the second expression onto – in the first. The result is 5(a+b)+6x–7.

Remember that substitution is a Blue Mode or Green Mode operation. If you are in another mode, substitution doesn’t work. Users often forget this and have a frustrating experience.

Substitution is both powerful and dangerous. It is safer in DragginMath than in some other software that provides this kind of behavior, but you must still be careful. It is so easy to do so much without envisioning all the consequences or noticing all the results. For example, enter 3x÷2; x=3. Drag x in the equation up onto ÷. The result is 3∗3÷2; x=3. No problem there. Now drag 3 in the equation up onto ÷ to reverse the process. Do you see what happened? This is exactly what you asked for but probably not what you wanted. What can be done about this?

You don’t have to drop a substitution on the root of a tree: you can drop it anywhere. So you can drag the 3 in the equation up and directly onto the x you actually want to change in the other expression. Then you must deliberately look for any other instances you want to change, instead of letting DragginMath find them all for you from the root.

This is not to deter you from using substitution, but please be attentive and cautious (and remember: Blue Mode or Green Mode, not Red or Magenta or Purple Mode). The equation you save may be your own.