What Is The Value Of Cosec 45 Degrees?
Cosec 45 degrees equals $\sqrt{2}$, and as a decimal that is about $1.4142$ (more precisely $1.41421356$). In symbols, $\csc 45^\circ = \sqrt{2}$. The angle $45^\circ$ can also be written as $\frac{\pi}{4}$ radians, so the same fact reads $\csc \frac{\pi}{4} = \sqrt{2}$.
The word cosecant just names the reciprocal of the sine function: $\csc\theta = \frac{1}{\sin\theta}$. Because $45^\circ$ lands in the first quadrant, where every ratio is positive, the answer carries a plus sign. This is one of the special-angle values worth keeping in memory, alongside the sines and cosines of $30^\circ$, $45^\circ$, and $60^\circ$.
Table: The value of cosec 45 degrees, at a glance.
Property | Value |
|---|---|
Exact value | $\sqrt{2}$ |
Decimal (4 dp) | $1.4142$ |
Angle in radians | $\frac{\pi}{4}$ |
Quadrant | I (positive) |
Reciprocal of | $\sin 45^\circ = \frac{\sqrt{2}}{2}$ |
How Do You Find Cosec 45 Degrees?
The cleanest route is the reciprocal rule, and a right triangle confirms it. Cosecant is the reciprocal of sine, so start from the sine of the angle.
$$\sin 45^\circ = \frac{\sqrt{2}}{2}$$
Now take the reciprocal and rationalise the denominator:
$$\csc 45^\circ = \frac{1}{\sin 45^\circ} = \frac{1}{\frac{\sqrt{2}}{2}} = \frac{2}{\sqrt{2}} = \frac{2}{\sqrt{2}}\cdot\frac{\sqrt{2}}{\sqrt{2}} = \frac{2\sqrt{2}}{2} = \sqrt{2}$$
The same value falls straight out of a right triangle. In any right triangle, cosecant of an angle is the hypotenuse divided by the side opposite that angle:
$$\csc\theta = \frac{\text{hypotenuse}}{\text{opposite}}$$
A $45^\circ$ angle lives in a 45-45-90 triangle, an isosceles right triangle whose two legs are equal. Take both legs equal to $1$. By the Pythagorean theorem the hypotenuse is $\sqrt{1^2 + 1^2} = \sqrt{2}$. The side opposite the $45^\circ$ angle is a leg of length $1$, so:
$$\csc 45^\circ = \frac{\text{hypotenuse}}{\text{opposite}} = \frac{\sqrt{2}}{1} = \sqrt{2}$$
Two methods, one answer. That is the double check every special-angle value should pass: the reciprocal-of-sine route and the triangle route must agree.
Where Does 45 Degrees Sit On The Unit Circle?
On the unit circle, the point for a given angle has coordinates $(\cos\theta, \sin\theta)$, and cosecant reads off as $\frac{1}{y}$, the reciprocal of the $y$-coordinate. The $45^\circ$ ray meets the circle at a point where the $x$ and $y$ coordinates are equal, because the angle splits the first quadrant exactly in half.
$$\left(\cos 45^\circ,\ \sin 45^\circ\right) = \left(\frac{\sqrt{2}}{2},\ \frac{\sqrt{2}}{2}\right) \approx (0.7071,\ 0.7071)$$
Cosecant is the reciprocal of that $y$-coordinate:
$$\csc 45^\circ = \frac{1}{y} = \frac{1}{\frac{\sqrt{2}}{2}} = \sqrt{2}$$
Since $y$ is positive throughout the first quadrant, so is its reciprocal, which is why cosec 45 degrees is positive.
Why Is Cosec 45 Degrees Equal To Sec 45 Degrees?
There is a neat symmetry hiding in this angle. Cosec 45 degrees and sec 45 degrees are the same number, both $\sqrt{2}$, and the reason is the cofunction relationship between sine and cosine.
The cofunction identity says $\csc\theta = \sec(90^\circ - \theta)$, because a cofunction of an angle equals the function of its complement.
The complement of $45^\circ$ is $90^\circ - 45^\circ = 45^\circ$. The angle is its own complement, so it is self-complementary.
Feeding that in: $\csc 45^\circ = \sec(90^\circ - 45^\circ) = \sec 45^\circ$.
That is why the two values coincide only at $45^\circ$. It is the single angle where the cofunction map sends a ratio to itself, the fixed point of the whole complementary-angle picture. You can see the same balance on the unit circle: at $45^\circ$ the $x$ and $y$ coordinates are equal, so sine equals cosine, and therefore their reciprocals, cosecant and secant, are equal too.
What Are The Cosecant Values Of The Other Special Angles?
Cosec 45 degrees is one entry in a short table worth knowing by heart. Each cosecant below is just one divided by the matching sine.
Table: Cosecant of the common first-quadrant angles, in degrees and radians.
Angle (degrees) | Angle (radians) | $\sin\theta$ | $\csc\theta = \frac{1}{\sin\theta}$ |
|---|---|---|---|
$0^\circ$ | $0$ | $0$ | undefined |
$30^\circ$ | $\frac{\pi}{6}$ | $\frac{1}{2}$ | $2$ |
$45^\circ$ | $\frac{\pi}{4}$ | $\frac{\sqrt{2}}{2}$ | $\sqrt{2} \approx 1.4142$ |
$60^\circ$ | $\frac{\pi}{3}$ | $\frac{\sqrt{3}}{2}$ | $\frac{2\sqrt{3}}{3} \approx 1.1547$ |
$90^\circ$ | $\frac{\pi}{2}$ | $1$ | $1$ |
Two things stand out. Cosecant is never between $-1$ and $1$, because it is one over a sine that never exceeds $1$ in size. And at $0^\circ$ it is undefined, because you cannot divide by $\sin 0^\circ = 0$. For the full grid across all four quadrants, see the trigonometric table, and for how these fit the wider set of ratios see the trigonometric ratios of specific angles.
Who Discovered Cosec 45 Degrees?
Nobody discovered the number $\sqrt{2}$ inside a formula one afternoon. The cosecant grew out of centuries of table-making, as astronomers tried to predict where the stars and planets would be. The ratio we now call cosecant was among the last of the six to be named, arriving with the mathematicians of the medieval Islamic world.
Two earlier figures set the stage:
Hipparchus of Nicaea (c. 190–120 BCE, Greece) built the first known table of chords, the distant ancestor of the sine table, and is often called the founder of trigonometry.
Aryabhata (476–550 CE, India) tabulated the half-chord, or jya, in his Aryabhatiya around 499 CE. The word jya travelled through Arabic into Latin and became our word "sine," the very function whose reciprocal is the cosecant.
Where Is Cosec 45 Degrees Used In The Real World?
The $45^\circ$ angle is everywhere a designer wants a balanced diagonal, and cosecant is the ratio that turns a horizontal run into the length of the slope.
Ramps and roof pitches: on a surface tilted at $45^\circ$, the sloped length is $\sqrt{2}$ times the horizontal run. A builder pricing decking or roofing for a $45^\circ$ pitch is really multiplying by cosec 45 degrees.
Forces on an incline: resolving a weight along a $45^\circ$ slope brings in $\sin 45^\circ$ and its reciprocal, so the same $\sqrt{2}$ shows up in mechanics problems about sliding blocks and tensioned cables.
Optics and prisms: a right-angle prism reflects light off a $45^\circ$ face, and the geometry of the light path uses the special-angle ratios directly.
Computer graphics: isometric and $45^\circ$ views, common in games and technical drawing, rely on the equal-coordinate point that also gives cosecant its value here.
Electrical engineering: the peak of an alternating current relates to its effective value through the same $\sqrt{2}$ factor that sits at $45^\circ$ on the sine wave.
One tidy number, $\sqrt{2}$, links a skate ramp, a beam of light, and the mains supply. Mathematics keeps reusing its best ideas across fields that look nothing alike.
What Are The Most Common Mistakes With Cosec 45 Degrees?
These three slips account for most wrong answers on reciprocal ratios, and each has a clean fix.
Taking the reciprocal of the wrong function.
Where it slips in:
A student sees the "co" in cosecant and pairs it with cosine, computing $\frac{1}{\cos 45^\circ}$ instead of $\frac{1}{\sin 45^\circ}$.
Don't do this:
Do not read cosecant as the reciprocal of cosine. That reciprocal is the secant.
The correct way:
Cosecant is the reciprocal of sine, secant is the reciprocal of cosine, and cotangent is the reciprocal of tangent. At $45^\circ$ the sine and cosine happen to be equal, so the error is invisible here, but at any other angle it produces the wrong number.
Forgetting to invert.
Where it slips in:
A student recalls $\sin 45^\circ = \frac{\sqrt{2}}{2}$ and writes that down as the cosecant, skipping the reciprocal step.
Don't do this:
Do not report $\frac{\sqrt{2}}{2} \approx 0.7071$. That is the sine, not the cosecant.
The correct way:
Flip it. $\csc 45^\circ = \frac{1}{\sin 45^\circ} = \frac{2}{\sqrt{2}} = \sqrt{2} \approx 1.4142$. A quick sanity check: cosecant is always at least $1$ in size, so any answer below $1$ must be wrong.
Working in the wrong angle mode.
Where it slips in:
A student types $45$ into a calculator set to radians, or looks for a "csc" key that most calculators do not have.
Don't do this:
Do not evaluate $\csc(45\text{ radians})$, and do not assume a dedicated cosecant button exists.
The correct way:
Set the calculator to degree mode for $45^\circ$ (or use $\frac{\pi}{4}$ in radian mode), then compute $1 \div \sin(45^\circ)$. The result reads $1.41421356\ldots$, which is $\sqrt{2}$.
Practice Problems On Cosec 45 Degrees
Work each one, then check against the answer that follows.
Write the exact value of $\csc 45^\circ$.
(Answer: $\sqrt{2}$.)Evaluate $\csc 45^\circ \times \sin 45^\circ$.
(Answer: $1$, since a value times its reciprocal is $1$.)Evaluate $\csc^2 45^\circ$.
(Answer: $(\sqrt{2})^2 = 2$.)Find $\csc 45^\circ + \sec 45^\circ$.
(Answer: $\sqrt{2} + \sqrt{2} = 2\sqrt{2} \approx 2.8284$.)A right triangle has a $45^\circ$ angle and the side opposite it measures $5$ cm. Find the hypotenuse.
(Answer: hypotenuse $=$ opposite $\times \csc 45^\circ = 5\sqrt{2} \approx 7.07$ cm.)Express $\csc 45^\circ$ as a secant of the complementary angle.
(Answer: $\sec(90^\circ - 45^\circ) = \sec 45^\circ$.)
Where Should You Go Next After Cosec 45 Degrees?
Cosec 45 degrees opens onto the wider world of reciprocal ratios and special angles. A few natural doors:
Reciprocal of sine. See how cosecant is built from sine across every angle, not just $45^\circ$, and why it is undefined wherever sine is zero.
Sin 45 degrees. The value this whole page inverts, derived from the same 45-45-90 triangle.
Cofunction identities. The rule that made cosec 45 degrees equal to sec 45 degrees, applied to every complementary pair.
Cosecant functions. The full graph and behaviour of the cosecant curve.
If your child is building these foundations, a live Bhanzu trainer teaches special-angle values starting from the "why", the triangle and the circle behind each number, in the Bhanzu trigonometry program.
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