Sparknotes Practice Math II Question

Test Preparation SAT Subject Test Preparation
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<li> An ellipse has equation 100x^2 + 25y^2 = 175. What is the length of the major axis of the ellipse?</li>
</ol>

<p>(A) 1.51
(B) 2.65
(C) 3.50
(D) 5.29
(E) 7.00</p>

<p>I chose B but it’s apparently C. The given explanation isn’t doing it for me; I tried to squeeze it into the format that y^2/a^2 - x^2/b^2 = 1 but ran into some problems. Anyone want to explain it?</p>

<p>Also, out of the whole practice test I ended up with 46/50 right, 800 on my first practice test. You think this is a pretty safe indication that with just some review and maybe a CB official practice test I’m good for Saturday?</p>

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<p>Divide by 175 because its in the form x^2/a^2+y^2/b^2=1.</p>

<p>So then a^2=175/100, a= around 1.32-1.33, so 2a = around 2.65, hmm thats werid I get B also…</p>

<p>They must have thought the forummla was x^2/a, not x^2/a^2…</p>

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<p>yea…im pretty sure that I am right…where did this question come from?</p>

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<p>sparknotes glitches a lot. the answer is D, 5.29 because:</p>

<p>when you divide both sides by 175 you get 100x^2/175 + 25y^2/175 = 1
you flip the constants around, giving you x^2/(175/100) + y^2/(175/25), or x^2/1.75 + y^2/7, so:
a^2 is 1.75 and b^2 is 7
you pick the bigger number, b^2, which is under the y
so, b = radical 7, which is the distance from the center to the end,
thus the length is 2*radical 7, or 5.29.</p>

<p>this is the answer shown in my test; the way you find out if the answer glitched or not is by looking at the explanation to see if it matches.</p>

<p>to check the answer, just divide the 5.29 by 2 again, then square it and youll get 7.</p>

<p>or dont square it, and use radical 7 as the y value. (0, radical 7) is the point, plug that into the original equation and youll get 175</p>

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<p>Oh crap you are <a href=“mailto:right...haha@myself”>right…haha@myself</a>. </p>

<p>Yea, I forgot that its gonna be 175/25 (I forgot the it can be vertical ellipse), which means that b^2=7, b=2.6ish, so 2b=5.2.</p>

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<p>oh and the reason you got 2.65 and not 5.29 is because you didnt multiply it by 2.</p>

<p>2.65 is the length from the center (0,0 in this case) to 1 end of the major axis because when you radicalize the 7 (y^2 / 7), you get a positive AND negative 2.65 (negative 2.65 squared is still 7)</p>

<p>so, the ends of the major axis are (0, -2.65) to (0, 2.65), and the length is 2*2.65</p>

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<p>Thanks crazy!</p>

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<p>if u do sparknotes it’s really helpful</p>

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<p>I think you should be good, since I am getting around your score give or take a point. In fact, I got 46 right 4 omitted (questions that didn’t logically work or that I forgot to go back to), and I feel fairly confident of an 800. You should too.</p>

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<p>Thanks da, I hope you’re right!</p>

<p>Good luck everyone!</p>