Answer
$\sqrt2$
Work Step by Step
RECALL:
(1) $\sin{\theta} = \dfrac{\text{opposite side}}{\text{hypotenuse}}$
(2) $\tan{\theta} = \dfrac{\text{opposite side}}{\text{adjacent side}}$
(3) $\cos{\theta} = \dfrac{\text{adjacent side}}{\text{hypotenuse}}$
(4) $\sec{\theta} = \dfrac{\text{hypotenuse}}{\text{adjacent}}$
(5) $\csc{\theta} = \dfrac{\text{hypotenuse}}{\text{opposite side}}$
Use formula (4) above.
Use the 45-degree angle of the triangle on the left to obtain:
$\sec{45^o} = \dfrac{\sqrt2}{1}=\sqrt2$
