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We can find an unknown side in a right-angled triangle when we know:
Example: Depth to the SeabedThe ship is anchored on the seabed. We know:
So we should be able to find the depth! But How? The answer is to use Sine, Cosine or Tangent! But Which One?Which one of Sine, Cosine or Tangent to use? To find out which, first we give names to the sides:
Now, for the side we already know and the side we are trying to find, we use the first letters of their names and the phrase "SOHCAHTOA" to decide which function:
Like this:
Example: Depth to the Seabed (Continued)Find the names of the two sides we are working on:
Now use the first letters of those two sides (Opposite and Hypotenuse) and the phrase "SOHCAHTOA" which gives us "SOHcahtoa", which tells us we need to use Sine: Sine: sin(θ) = Opposite / Hypotenuse Now put in the values we know: sin(39°) = d / 30 And solve that equation! But how do we calculate sin(39°) ... ?
sin(39°) = 0.6293... So now we have: 0.6293... = d / 30 Now we rearrange it a little bit, and solve:
Start with:0.6293... = d / 30 Swap sides:d / 30 = 0.6293... Multiply both sides by 30:d = 0.6293... x 30 Calculate:d = 18.88 to 2 decimal places The depth the anchor ring lies beneath the hole is 18.88 m Step By StepThese are the four steps to follow:
ExamplesLet’s look at a few more examples:
Example: find the height of the plane.We know the distance to the plane is 1000 What is the plane's height? Careful! The 60° angle is at the top, so the "h" side is Adjacent to the angle!
Start with:cos 60° = h/1000 Swap:h/1000 = cos 60° Calculate cos 60°:h/1000 = 0.5 Multiply both sides by 1000:h = 0.5 x 1000 h = 500 The height of the plane = 500 meters
Example: Find the length of the side y:
Start with:tan 53° = y/7 Swap:y/7 = tan 53° Multiply both sides by 7:y = 7 tan 53° Calculate:y = 7 x 1.32704... y = 9.29 to 2 decimal places Side y = 9.29
Example: Radio MastThere is a mast that is 70 meters high. A wire goes to the top of the mast at an angle of 68°. How long is the wire?
The unknown length is on the bottom (the denominator) of the fraction! So we need to follow a slightly different approach when solving:
Start with:sin 68° = 70/w Multiply both sides by w:w × (sin 68°) = 70 Divide both sides by "sin 68°":w = 70 / (sin 68°) Calculate:w = 70 / 0.9271... w = 75.5 m to 1 decimal place The length of the wire = 75.5 m 258, 1504, 1505, 1506, 1507, 2346, 2347, 2348, 3935, 248 Copyright © 2022 Rod Pierce A right triangle (American English) or right-angled triangle (British), or more formally an orthogonal triangle, formerly called a rectangled triangle[1] (Ancient Greek: ὀρθόσγωνία, lit. 'upright angle'),[2] is a triangle in which one angle is a right angle (that is, a 90-degree angle), i.e., in which two sides are perpendicular. The relation between the sides and other angles of the right triangle is the basis for trigonometry. The side opposite to the right angle is called the hypotenuse (side c in the figure). The sides adjacent to the right angle are called legs (or catheti, singular: cathetus). Side a may be identified as the side adjacent to angle B and opposed to (or opposite) angle A, while side b is the side adjacent to angle A and opposed to angle B. If the lengths of all three sides of a right triangle are integers, the triangle is said to be a Pythagorean triangle and its side lengths are collectively known as a Pythagorean triple. Principal propertiesAreaAs with any triangle, the area is equal to one half the base multiplied by the corresponding height. In a right triangle, if one leg is taken as the base then the other is height, so the area of a right triangle is one half the product of the two legs. As a formula the area T is T = 1 2 a b {\displaystyle T={\tfrac {1}{2}}ab}where a and b are the legs of the triangle. If the incircle is tangent to the hypotenuse AB at point P, then denoting the semi-perimeter (a + b + c) / 2 as s, we have PA = s − a and PB = s − b, and the area is given by T = PA ⋅ PB = ( s − a ) ( s − b ) . {\displaystyle T={\text{PA}}\cdot {\text{PB}}=(s-a)(s-b).}This formula only applies to right triangles.[3] AltitudesAltitude of a right triangleIf an altitude is drawn from the vertex with the right angle to the hypotenuse then the triangle is divided into two smaller triangles which are both similar to the original and therefore similar to each other. From this:
In equations, f 2 = d e , {\displaystyle \displaystyle f^{2}=de,} (this is sometimes known as the right triangle altitude theorem) b 2 = c e , {\displaystyle \displaystyle b^{2}=ce,} a 2 = c d {\displaystyle \displaystyle a^{2}=cd}where a, b, c, d, e, f are as shown in the diagram.[5] Thus f = a b c . {\displaystyle f={\frac {ab}{c}}.}Moreover, the altitude to the hypotenuse is related to the legs of the right triangle by[6][7] 1 a 2 + 1 b 2 = 1 f 2 . {\displaystyle {\frac {1}{a^{2}}}+{\frac {1}{b^{2}}}={\frac {1}{f^{2}}}.}For solutions of this equation in integer values of a, b, f, and c, see here. The altitude from either leg coincides with the other leg. Since these intersect at the right-angled vertex, the right triangle's orthocenter—the intersection of its three altitudes—coincides with the right-angled vertex. Pythagorean theoremThe diagram for Euclid's proof of the Pythagorean theorem: each smaller square has area equal to the rectangle of corresponding color.The Pythagorean theorem states that:
This can be stated in equation form as a 2 + b 2 = c 2 {\displaystyle \displaystyle a^{2}+b^{2}=c^{2}}where c is the length of the hypotenuse, and a and b are the lengths of the remaining two sides. Pythagorean triples are integer values of a, b, c satisfying this equation Inradius and circumradiusThe radius of the incircle of a right triangle with legs a and b and hypotenuse c is r = a + b − c 2 = a b a + b + c . {\displaystyle r={\frac {a+b-c}{2}}={\frac {ab}{a+b+c}}.}The radius of the circumcircle is half the length of the hypotenuse, R = c 2 . {\displaystyle R={\frac {c}{2}}.}Thus the sum of the circumradius and the inradius is half the sum of the legs:[8] R + r = a + b 2 . {\displaystyle R+r={\frac {a+b}{2}}.}One of the legs can be expressed in terms of the inradius and the other leg as a = 2 r ( b − r ) b − 2 r . {\displaystyle \displaystyle a={\frac {2r(b-r)}{b-2r}}.}CharacterizationsA triangle ABC with sides a ≤ b < c {\displaystyle a\leq b<c} , semiperimeter s, area T, altitude h opposite the longest side, circumradius R, inradius r, exradii ra, rb, rc (tangent to a, b, c respectively), and medians ma, mb, mc is a right triangle if and only if any one of the statements in the following six categories is true. All of them are of course also properties of a right triangle, since characterizations are equivalences. Sides and semiperimeter
Angles
Area
Inradius and exradii
Altitude and mediansThe altitude of a right triangle from its right angle to its hypotenuse is the geometric mean of the lengths of the segments the hypotenuse is split into. Using Pythagoras' theorem on the 3 triangles of sides (p + q, r, s ), (r, p, h ) and (s, h, q ),( p + q ) 2 = r 2 + s 2 p 2 + 2 p q + q 2 = p 2 + h 2 ⏞ + h 2 + q 2 ⏞ 2 p q = 2 h 2 ∴ h = p q {\displaystyle {\begin{aligned}(p+q)^{2}\;\;&=\quad r^{2}\;\;\,+\quad s^{2}\\p^{2}\!\!+\!2pq\!+\!q^{2}&=\overbrace {p^{2}\!\!+\!h^{2}} +\overbrace {h^{2}\!\!+\!q^{2}} \\2pq\quad \;\;\;&=2h^{2}\;\therefore h\!=\!{\sqrt {pq}}\\\end{aligned}}}
Circumcircle and incircle
Trigonometric ratiosThe trigonometric functions for acute angles can be defined as ratios of the sides of a right triangle. For a given angle, a right triangle may be constructed with this angle, and the sides labeled opposite, adjacent and hypotenuse with reference to this angle according to the definitions above. These ratios of the sides do not depend on the particular right triangle chosen, but only on the given angle, since all triangles constructed this way are similar. If, for a given angle α, the opposite side, adjacent side and hypotenuse are labeled O, A and H respectively, then the trigonometric functions are sin α = O H , cos α = A H , tan α = O A , sec α = H A , cot α = A O , csc α = H O . {\displaystyle \sin \alpha ={\frac {O}{H}},\,\cos \alpha ={\frac {A}{H}},\,\tan \alpha ={\frac {O}{A}},\,\sec \alpha ={\frac {H}{A}},\,\cot \alpha ={\frac {A}{O}},\,\csc \alpha ={\frac {H}{O}}.}For the expression of hyperbolic functions as ratio of the sides of a right triangle, see the hyperbolic triangle of a hyperbolic sector. Special right trianglesThe values of the trigonometric functions can be evaluated exactly for certain angles using right triangles with special angles. These include the 30-60-90 triangle which can be used to evaluate the trigonometric functions for any multiple of π/6, and the 45-45-90 triangle which can be used to evaluate the trigonometric functions for any multiple of π/4. Kepler triangleLet H, G, and A be the harmonic mean, the geometric mean, and the arithmetic mean of two positive numbers a and b with a > b. If a right triangle has legs H and G and hypotenuse A, then[15] A H = A 2 G 2 = G 2 H 2 = ϕ {\displaystyle {\frac {A}{H}}={\frac {A^{2}}{G^{2}}}={\frac {G^{2}}{H^{2}}}=\phi \,}and a b = ϕ 3 , {\displaystyle {\frac {a}{b}}=\phi ^{3},\,}where ϕ {\displaystyle \phi } is the golden ratio 1 + 5 2 . {\displaystyle {\tfrac {1+{\sqrt {5}}}{2}}.\,} Since the sides of this right triangle are in geometric progression, this is the Kepler triangle. Thales' theoremMedian of a right angle of a triangleThales' theorem states that if A is any point of the circle with diameter BC (except B or C themselves) ABC is a right triangle where A is the right angle. The converse states that if a right triangle is inscribed in a circle then the hypotenuse will be a diameter of the circle. A corollary is that the length of the hypotenuse is twice the distance from the right angle vertex to the midpoint of the hypotenuse. Also, the center of the circle that circumscribes a right triangle is the midpoint of the hypotenuse and its radius is one half the length of the hypotenuse. MediansThe following formulas hold for the medians of a right triangle: m a 2 + m b 2 = 5 m c 2 = 5 4 c 2 . {\displaystyle m_{a}^{2}+m_{b}^{2}=5m_{c}^{2}={\frac {5}{4}}c^{2}.}The median on the hypotenuse of a right triangle divides the triangle into two isosceles triangles, because the median equals one-half the hypotenuse. The medians ma and mb from the legs satisfy[8]: p.136, #3110 4 c 4 + 9 a 2 b 2 = 16 m a 2 m b 2 . {\displaystyle 4c^{4}+9a^{2}b^{2}=16m_{a}^{2}m_{b}^{2}.}Euler lineIn a right triangle, the Euler line contains the median on the hypotenuse—that is, it goes through both the right-angled vertex and the midpoint of the side opposite that vertex. This is because the right triangle's orthocenter, the intersection of its altitudes, falls on the right-angled vertex while its circumcenter, the intersection of its perpendicular bisectors of sides, falls on the midpoint of the hypotenuse. InequalitiesIn any right triangle the diameter of the incircle is less than half the hypotenuse, and more strongly it is less than or equal to the hypotenuse times ( 2 − 1 ) . {\displaystyle ({\sqrt {2}}-1).} [16]: p.281 In a right triangle with legs a, b and hypotenuse c, c ≥ 2 2 ( a + b ) {\displaystyle c\geq {\frac {\sqrt {2}}{2}}(a+b)}with equality only in the isosceles case.[16]: p.282, p.358 If the altitude from the hypotenuse is denoted hc, then h c ≤ 2 4 ( a + b ) {\displaystyle h_{c}\leq {\frac {\sqrt {2}}{4}}(a+b)}with equality only in the isosceles case.[16]: p.282 Other propertiesIf segments of lengths p and q emanating from vertex C trisect the hypotenuse into segments of length c/3, then[4]: pp. 216–217 p 2 + q 2 = 5 ( c 3 ) 2 . {\displaystyle p^{2}+q^{2}=5\left({\frac {c}{3}}\right)^{2}.}The right triangle is the only triangle having two, rather than one or three, distinct inscribed squares.[17] Given h > k. Let h and k be the sides of the two inscribed squares in a right triangle with hypotenuse c. Then 1 c 2 + 1 h 2 = 1 k 2 . {\displaystyle {\frac {1}{c^{2}}}+{\frac {1}{h^{2}}}={\frac {1}{k^{2}}}.}These sides and the incircle radius r are related by a similar formula: 1 r = − 1 c + 1 h + 1 k . {\displaystyle \displaystyle {\frac {1}{r}}=-{\frac {1}{c}}+{\frac {1}{h}}+{\frac {1}{k}}.}The perimeter of a right triangle equals the sum of the radii of the incircle and the three excircles: a + b + c = r + r a + r b + r c . {\displaystyle a+b+c=r+r_{a}+r_{b}+r_{c}.}See also
References
External links
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