Solving an Equation Containing a Radical, Part 1 of 2, from Thinkwell Precalculus

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Category:

Education

Tags:

• ed
• burger
• square
• root
• quadratic
• equation
• algebra

[JeremyNWatson]

I'm no expert on the matter, but isn't this wrong???? All positive real numbers have two square roots, one positive and one negative. So the square roots of 25 are 5 AND -5, and the square roots of 9 are 3 and -3. Therefore both possibilities are true. Did I miss something here?

[ThinkwellVids]

@JeremyNWatson You've got to plug in the answers to see which ones work. They don't all always work.

[blazingjavi]

@Thinkwellvids but couldn't you just remove the square root by squaring both sides so 5=5?

[ThinkwellVids]

@blazingjavi Not sure what part of the problem you are referring to in order to get to 5=5. Let me know and I'll see if I can help.

[blazingjavi]

@ThinkwellVids when your checking your answer to see if x= -5. You just remove the square root on 15-2x that way the equation ends up x^2=15-2x, and that way x=-5

[ThinkwellVids]

@blazingjavi The problem is when you check your answer. Remember when you simplify x^2=15-2x you get (x+5)(x-3), which gives you x=-5 and x=3. But when you go plug in each of these numbers into the original equation, you'll find that -5 does not work but 3 does.

[iwantram3n]

I really like how you not only write the work clearly on a nice big surface but you also include your face on the top corner. That makes it feel more personal and like a 1 on 1 virtual lesson.

However, I think this material is more Algebra 1 or 2 than Precalculus, but that may be different depending on which school you go to.

[ThinkwellVids]

@kooldude917 Professor Burger loves teaching math and I think it really shows.

[ixionike]

In fact, there's a theorem you can apply when you have an equation of a form 1x^2+bx+c=0, which says that x1*x2=c and x1+x2= -b, where x1 and x2 are the roots. So you just come up with the numbers that satisfy this condition. That's very easy for small integers like ones in this problem.

[adunolajr]

but isnt the sqareroot of 25, -5 also, because it's +or -,( -5^2) is 25....

[kooldude917]

u r very good at this are you an expert