81^2 = 18 ^2 + 80^
6561 = 324 + 6400
6561 = 6724
<span>It is not a right angle, since 18^2 + 80^2 does not equal 81^2.
So,
Using </span><span>The law of cosines:
</span>The largest angle C is opposite to the side c.
Therefore,
<span>c^2 = a^2 + b^2 -2ab cosC </span>
<span>Here,
c^2 < (a^2 + b^2)
cosC must be positive which makes C an acute angle.</span>
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