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Let alt=blt=c be the lengths of the side...

Let `alt=blt=c` be the lengths of the sides of a triangle. If `a^2+b^2

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`a^(2) + b^(2) lt c^(2)`
`rArr a^(2) + b^(2) lt a^(2) + b^(2) - 2ab cos C`
`rArr cos C lt 0`
Hence, C is an obtuse angles
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