21/x + 1 = 7/2 ALERT: Many Math students get this step WRONG!

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How to solve proportion problems in algebra using the cross-product.
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27 июл 2023

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Комментарии : 44

@williamwoods2547 Год назад

Well done as usual. My great aunt, born 1877, graduated from the University of Missouri before 1900. She taught math in private high schools and Christian colleges for women during the first two decades of the 20th century and loved math because it was, as she said, very logical, and nothing to be afraid of.

@robertakerman3570 Год назад

Very clever yet basic prob

@freeguy77 Год назад

First university in the whole world (1908) to implement a journalism school. For better or for worse! Also claimed credit for being the first university to start the homecoming alumni tradition for the big football game (vs, Kansas) in 1911!

@glaucouswingedgull Год назад

The same error you explained was the same error you made in describing the original problem by omitting the use of parenthesis in the original problem description.

@timwong8556 Год назад

7x +7 =42 7x=35 X =5

@PrepworksEducation Год назад

Very useful method! Pops up in geometry similarity problems as well.

@davedeatherage4902 Год назад

Thank you John, your teaching manner explanation, make this understandable.

@cadfael4598 Год назад

There is another way…cross multiply by 1/7 and by x+1. That gives us 21/7 = (x+1)/2, OR 3 = (x+1)/2. Then multiply both sides by 2 so 6 = x+1 and therefore x = 6-1. =5 QED

@devonwilson5776 10 месяцев назад

Greetings. The answer is definitely 5. That is X=5. From the expression given we have determined that 42=7X +7 by cross multiplying. From this expression we find that 7X =35 and dividing each side by 7 leaves us with X =5.

@freeguy77 Год назад

7 is 1/3rd of the opposite numerator's 21. So, multiplying by 3 / 3 (1) on the right side gives the new equivalent equations: 21 / (X + 1) = 21 / 6. So, the denominators must also now be equal. X + 1 = 6, so X = 5. Because the 21 and 7 are different by an easy factor of 3, you don't need to cross-multiply at all! 7X + 7 = 42, so 7X =35, so X = 5 by the cross-multiply method, which is usually necessary, if there is no easy factor to make equivalent on one side to the other side's numerator or denominator.

@MrMousley 8 месяцев назад

21/(X + 1) = 7/2 multiply both sides by 2 and (X + 1) 21 x 2 = 7 (X+ 1) 42 = 7X + 7 subtract 7 from both sides 42 - 7 = 7X 35 = 7X divide both sides by 7 5 = X

@daviddempsey8721 Год назад

The cross product is usually the term for a matrix multiplication function.

@abhigyanshree5676 Год назад

21/x+1 = 7/2 3/x+1 = 1/2 [canceling 21 and 7] 6 = x+1 [by cross multiplication] x = 5 [subtracting 1 from both sides]

@user-ky5dy5hl4d Год назад

Another method: Flip the denominator with the numerator on both sides of the proportion and solve for X.

@panlomito 7 месяцев назад

No calculator needed: 7/2 . ( x + 1 ) = 21 so x + 1 = 21 . 2/7 = 3.2 = 6 and x = 6 - 1 = 5

@elai3147 Год назад

hopefully students know the product of 7 and x+1 is not 7x+1

@BansheeBunny 9 месяцев назад

I don't think I did this the way you're supposed to. 21\7*2-1=5

@thomashughes4859 Год назад

In my head, I did this. 21/3=7, & 6/3=2. X-multiply!

@marymetzger7171 Год назад

@suraponwarrarak2239 Год назад

So simple

@baselinesweb 11 месяцев назад

I find it hard to believe that many will get this wrong.

@1234larry1 Месяц назад

Lets say we use the term “cross multiply” instead of “cross product” unless we’re talking about multiplying vectors lol

@billh5923 11 месяцев назад

21/(x+1) = 21/6 x = 5

@DayanandPandey-ns5qc 8 месяцев назад

Ans. X ka man 5

@tonidozier4573 10 месяцев назад

X = 5, I cross multiplied

@Herlongian Год назад

21 is 3 times more than 7. So x+1 must be 3 times more than 2. Rewrite the equation as x+1=6 and go from there.

@freeguy77 Год назад

I think it could be X = 5 !!!

@lawrencejelsma8118 Год назад

That's the geometric proportions way people have to do it in similar triangles. I prefer that way also knowing 7 divides into 21 by 3 the denominator must also divide into a proportional denominator of 3; that is, 7 is to 2 as 21 is to 6 proportions.

@poeda6637 Год назад

What you have to do to make 21 from 7? Multiply by 3. So (x+1) multiplied by 3 is 2 multiplied by 3 = 6. So X+1= 6 so x=5

@PaulBrown-il3wl Год назад

X = 5

@silverhammer7779 11 месяцев назад

B+? Ignoring the fact that (7) (X + 1) = (7X + 7) deserves an F-.

@harpleblues 11 месяцев назад

Just another reason why PEMDAS doesn’t work.

@vespa2860 Год назад

Your title is misleading - suggests (21/x) +1 = 7/2 .............. Pemdas or not pemdas

@prof.lsteele6570 Год назад

Ok, paused at the beginning to make a fool of myself as usual! 7/2 = 3.5 21 / 2 = 10.5 10.5 - 1 = 9.5 which is x. Insert mocking comments below 👇

@prof.lsteele6570 Год назад

I wonder why I divided by 2 and not 3.5 to begin with.... Might be the wine. Also the 13hr shift I just worked, my 13th in a row. God I wish I was an actual student.

@prof.lsteele6570 Год назад

Huh, his way of solving it was really cool. For little old me the cross product is when you multiply two vectors and get a perpendicular vector. Like vec(1, 0, 0) x vec(0, 1, 0) is vec(0, 0, 1) but I never heard of this fancy thing of doing it with fractions. Hey-ho.

@sherlock2drum Год назад

Bringing attention to math is a good thing, however, please do not give a B+ for a wrong answer. The nice part of math is it is either the correct answer or not, there is no grey area. Rewarding students for wrong answers is not a good teaching method in my opinion. the rest of the video is great.

@nobodyshome 5 месяцев назад

quit showing the wrong way to do the problem and just show the correct way so we don't confuse it in the future... SMH poor teaching method and you should know that

@nobodyshome 5 месяцев назад

I understand simplifying to the n'th degree for everyone but you repeat yourself and talk too much before even beginning to solve the problems on all your videos... Sorry your style is just not good for me. I am glad there are others... maybe in the future though give up the cute chat and get to the point... we are spending hours on this stuff already we don't need to hear you giving away smilies and stars