A) \[\frac{4}{5}\]
B) \[\frac{3}{5}\]
C) \[\frac{2}{5}\]
D) \[\frac{2}{3}\]
Correct Answer: A
Solution :
(a): \[\phi +\theta +90{}^\circ =180{}^\circ \] \[\Rightarrow \] \[\phi +\theta =90{}^\circ \] \[\Rightarrow \] \[\phi =90{}^\circ -\theta \] \[\Rightarrow \] \[\cos \phi =\cos (90{}^\circ -\theta )\] \[\Rightarrow \] \[\cos \phi =\sin \theta \] \[\Rightarrow \] \[\cos \phi =\frac{4}{5}\]
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