If $sinalpha=4/5$ and $alpha$ is obtuse angle, what is $cosalpha$ ?

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Answer 1 · 2017-03-02T16:30:53

$cosalpha=-3/5$

As $alpha$ is obtuse, it lies in second quadrant and $cosalpha$ will be negative.

Now $sinalpha=4/5$ and therefore

$cosalpha=-sqrt(1-(4/5)^2)=-sqrt(1-16/25)=-sqrt(9/25)=-3/5$

If sinalpha=4/5sinalpha=4/5 and alphaalpha is obtuse angle, what is cosalphacosalpha?