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# Quadratic and exponential word problems — Harder example

Watch Sal work through a harder Quadratic and exponential word problem.

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• sal am totally confuse even by rewatching it over and over.i rather skip this kind of question in SAT
• you can get the the decrease in the amount of subscribers for each 10 cent increase (for each choice ) then multiply the remaining amount with the the its selling cost.
For example,
10.65\$ has a 13.5 cents increase from the original 9.30\$ ... 13.5 x 20 = 270 ( the decrease in amount of subscribers) 2400-270 = 2130 ( the remaining amount of subscribers after the increase) 10.65\$ x 2130 = 22,684.5 \$ ( the revenue)
If you solved all the choices with the same technique you will find that
A) 1.35\$ x 2400 = 3240 \$
B) 9.30\$ x 2400 = 22,320 \$
C) 10.65\$ x 2130 = 22,684.5 \$
D) 22.80\$ x -300 = -6840 \$ ( not only will the company not gain any revenue ,but also they will lose 6840\$ )
• This question can be answered using common sense by simply looking at the choices :
The first choice gives the option of \$ 1.35 which will be too less as the costs won't be recovered.
The second choice gives the option of \$9.30 which is no change and the question says the prices can be increased.
The third choice gives the option of \$ 10.65 which is a fair increase.
And, the fourth choice gives the option of \$ 22.80 which is extremely high and there won't be any more subscribers.
• Sometimes math can be surprising, it's best not to take chances unless your under extreme time pressure.
• don't we only get like 50 minutes...
• he did this incredibly slowly. A better equation would be R(x) = (9.3 + 0.1x)(2400-20x), where R is equal to price (9.3 + 0.1x) multiplied by subscribers (2400-20x) and x is equal to the number of \$0.1 increases to the price. This can be quickly punched into a calculator where the vertex can be found, multiplied by 0.1 and then added to the original price.
• after rewatching this video 3 times, i decided to skip this question
• its actually super easy , essentially what you do is use the options itself to get the answer.. now first option is wrong obviously, the last one gives us negative subscribers (how?, well subtract 9.3 from it and divide it by 0.1 and then multiply it by 20 (why did we do that? because if we subtract we get the amount that has been increased and once we divide with 0.1 we get how much it has been increased wrt 0.1 so we can directly multiply with 20 to see how many subscribers we have gained or lost)) if we repeat this process for 2 and 3 we see there is a slight profit for the third option.. YES, u can skip it and attempt it in the last if u want 800
• I am really confused about this, on the real SAT does it actually format like this? The most difficult thing is interpreting the word problems into function, is there a easier way to do this?
• Anybody else use the derivative of the function & set it equal to zero? The Calculus method is much more simple if you asked me..
• Yes, I did the same by using maxima
(1 vote)
• Why does he divide by .10
• same thing i was thinking. He's supposed to multiply by 0.10 not divide. I'm so confused.
• At , how is 20 divided by .10? In order to cancel out .10, wouldn't you multiply by .10, and get 2 instead of 200?
• To cancel out .10, you need to multiply 20 by the reciprocal of .10, which is 10. Its easier to see this with fractions. .10=1/10, so the reciprocal is 10/1.