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Bunuel
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It's really easy if you know the law of cosine, which I don't know if that's expected for the gmat. c^2=a^2+b^2-2abcosC.

Not the most rigorous, but... expand S^2 (would work for U^2 and T^2 too), p^2=r^2-2rq+q^2 notice this resembles the above law of cosine expression. Notice also since r and q are positive, since they're magnitudes, then -2rq is negative. Therefore, one can infer that cos<90, as it would be 0 if 90deg and >0 if greater than 90deg.
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The best way to solve this question is knowing that :

s^2 + t^2 < u^2 --> Obtuse

s^2 + t^2 = u^2 --> Right Angled

s^2 + t^2 > u^2 --> Acute

So here we just try to find out s^2+t^2 - u^2 = r^2 - (p-q)^2

which can be written as (r-p+q)*(r+q-p) --> This is always positive because the p,q,r are sides of triangle and sum of two sides is always greater than 3rd side.

so in effect we just achieved s^2+t^2 > u^2. So the triangle is acute! Option A is correct answer.
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Can anyone explain in a different way how A is attained?
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Hello from the GMAT Club BumpBot!

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