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A little trigby mercury
Tags: trig 
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#1
Mar2603, 04:13 AM

P: n/a

this is a question that was part of an mcq test.
and i did'nt have any clue as to how to begin! finally i just tried plugging in values for A,B,C to see if they worked. for ex. i put A=B=C=60 (degrees) equilateral and so on..but i'd like to know the logical way to do it... if A,B,C are the vertices of a triangle, and if cosBcosC + sinAsinBsinC = 1 is the triangle a) equilateral b) isosceles c) right angled isosceles d) right angled but not isosceles e) none of the above. 


#2
Mar2603, 05:02 AM

P: 115

Right, I'm assuming you know sin^2 +cos^2 = 1.
Well you can see this is nearly similar, and almost exactly the same if B = C. So there's two angles the same. So now cos^2 + sinA sin^2 = 1 Which means sinA must be 1. Therefore A can only take value of 90 degrees  right angle (450...etc useless here) 180  90..other two angles combine as total 90 degrees, so they must be 45 each since you've already found they're the same. Which leaves you a right angled isosceles. 


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