You probably remember in elementary school when you used to use inequality symbols and the teacher would tell you to think of them as little alligators trying to eat the bigger number. Ex. 1<10 One is less than ten.
Well we're in high school now, and the inequality problems are no longer as simple :(
This is how an inequality problem looks now: -10x-20<5x+25
So how do you solve this? Let's take a look at it:
Step 1) Subtract 5x from both sides to get -> -15x-20<25
Step 2) Add 20 to both sides to get -> -15x<45
Step 3) Divide both sides by -15 to get -> x<-45
So based on our work, x should be less than -45, so let's pick a number less than -45 and plug it into the inequality to make sure its correct. Let's say we pick the number -46, plug it in -> -10(-46)-20<5(-46)+25 which equals to 440<-205. Woah! Wait a minute, that doesn't seem right! We must have made a mistake in our work, let's go back and check. Aha! It seems that we didn't switch the inequality symbol once we divided by -15.
So why must we flip the sign whenever we multiply or divide by a negative number?
Well let's look at a number line to help us with it:
Okay so lets say you have -2x<10, whenever you multiply or divide by a negative number (in this case we are dividing by -2), it's as if you flipped the number line, so this means that 10 turns to -5 (and -2 turns to 1) and a negative number is obviously less than a positive number. Thus, the inequality sign will no longer be facing the now -5, it will turn around and face the 1. The way i like to think of it is whenever you multiply or divide by a negative number, the little inequality symbol gets scared and turns away! :D
Hopefully you understood this concept, and as a reward for your perseverance and patience in reading this post, here's a picture of a kitty! -->
Follow the kitty's example and take a well deserved nap, they're awesome! :D
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