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Ask iit jee aieee pet cbse icse state board community Community Discussion Question: find angle plzzzzzzz
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sanchay_1991 (201)

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In a ABC , if a4 + b4 + c4 = 2c2(a2 + b2) , then  mC is equal to
 
(a) 600                       (b) 1350
 
(c)  900                      (d) 750
    
ankurgupta91 (828)

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cosine rule: c^2 = a^2 + b^2 -2abcosC
a^4 + b^4 = 2c^2(a^2+b^2 ) -c^4
a^4 + b^4 = c^2 ( 2a^ +2b^2 -c^2)
putting the value of c^2
a^4+b^4 = (a^2 + b^2 -2abcosC)(a^2+b^2+2abcosC)
a^4 + b^4=(a^2+ b^2)^2 - (2abcosC)^2
2a^2b^2 = 4a^2b^2cos^2C
cos C = +- 1/sqroot of 2
thus, angle C is either 45 or 135.
so the ans is (B)
hope u got it nw
thanks................

nobody is perfect......i m nobody..............
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sanchay_1991 (201)

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thanks ankur ur answer is absolutely correct
 
can i ask u a question
r u student of class 11 or 12 or doing college
reply if u have time otherwise its no special
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chirag (384)

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a^4+b^4+c^4=2c^2(a^2+b^2)
 
a^4+b^4+c^4-2c^2(a^2+b^2)=0
 
adding 2*a^2*b^2 both sides,
 
a^4+b^4+c^4-2c^2(a^2+b^2)+2*a^2*b^2=2*a^2*b^2
 
(a^2+b^2-c^2)^2=2*a^2*b^2
 
taking sqrt both sides,
 
(a^2+b^2-c^2)=+2*a*b.............(i)
 
or
 
(a^2+b^2-c^2)=-2*a*b............(ii)
 
using cosine rule,
 
cos(c)= b^2+a^2-c^2/2ab...................(iii)
 
Substituting (i) and (ii)
 
we get,
 
cos c= 45 or 135
 
therefore ans(B)
 



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