# A weighted inequality involving the sides of a triangle

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1 CREATIVE MATH. & INF. 19 (2010), No. 2, Online version available at Print Edition: ISSN X Online Edition: ISSN X A weighted inequality involving the sides of a triangle JIAN LIU ABSTRACT. A new weighted inequality involving the sides of a triangle is proved by applying the method of the Difference Substitution, and, as an application, a geometric inequality involving an interior point of the triangle is derived. Finally, two related conjectures which are checked by computer are put forward. 1. INTRODUCTION The earliest weighted inequality concerned with sides of a triangle appeared in J. Wolstenholme s book [1], and it is stated as follows: Let ABC be a triangle with sides a, b, c and let x, y, z be arbitrary real numbers. Then (x y)(x z)a 2 (y z)(y x)b 2 (z x)(z y)c 2 0, (1.1) with inequality if and only if x = y = z. Wolstenholme inequality (1.1) is one of the most important inequalities for the triangle, it has various equivalent versions and generalizations. The following important weighted inequality is actually its corollary: (xa yb zc) 2 (2bc 2ca 2ab a 2 b 2 c 2 )(yz zx xy). (1.2) with equality if and only if x : y : z = (b c a) : (c a b) : (a b c). Besides inequality (1.1) and (1.2), in literature (e.g [2], [3]) we see a few weighted inequalities involving the sides of a triangle. In 1992, the author established the following inequality with positive weights (see [4], [5]): s a x s b y s c z (xa yb zc)s yza zxb xyc, (1.3) s = (a b c)/2 and x, y, z are arbitrary positive real numbers. Equality holds in (1.3) if and only if x = y = z. Later, the author [6] obtained again results involving acute triangles and arbitrary real numbers (xa yb zc) 2 6 [a(s a)yz b(s b)zx c(s c)xy], (1.4) with equality if and only if the acute triangle ABC is equilateral and x = y = z. The aim of this paper is to establish a new weighted inequality for arbitrary triangles, and then apply it and some simple well known results to deduce a new geometric inequality involving an interior point of a triangle. Finally, we propose two related conjectures. Theorem 1.1. Let ABC be a triangle with sides a, b, c and let x, y, z be non-negative real numbers among which at least one is not zero. Then y z a(s a) z x b(s b) x y 12(yz zx xy) c(s c) xa 2 yb 2 zc 2, (1.5) s =(a b c)/2. Equality holds if and only if triangle ABC is equilateral and x=y =z. 2. THE PROOF OF THE THEOREM It is easy to see that the inequality (1.5) may be transformed into to the following ternary quadratic inequality: p 1 x 2 p 2 y 2 p 3 z 2 yzq 1 zxq 2 xyq 3, (2.6) and we have already known that inequality (2.6) holds for arbitrary real numbers x, y, z if and only if (e.g see [8], [7]): p 1 0, p 2 0, p 3 0, 4p 2 p 3 q 1 0, 4p 3 p 1 q 2 2 0, 4p 1 p 2 q 2 3 0, and 4p 1 p 2 p 3 (p 1 q 2 1 p 2 q 2 2 p 3 q 2 3 q 1 q 2 q 3 ) 0. (2.7) However, inequality (1.5) actually holds for non-negative real numbers x, y, z only, but it may not be valid for some real numbers x, y, z. So, we can not apply the conditions which endures that (2.6) holds for arbitrary real numbers x, y, z to prove (1.5). On the other hand, at the moment we don t know what are the conditions of inequality (2.6) holding for all positive real numbers x, y, z. Therefore, it is not easy to prove our theorem. We find out that our theorem can be proved by employing the method of the Difference Substitution (see [10], [9]). Received: ; In revised form: ; Accepted: Mathematics Subject Classification. 51M16, 26D15. Key words and phrases. Triangle, side, positive numbers, difference substitution, interior point. 160

2 A weighted inequality involving the sides of a triangle 161 Proof. We set s a = u, s b = v, s c = w, then a = v w, b = w u, c = u v, u > 0, v > 0, w > 0, and the inequality (1.5) is equivalent to the following: y z u(vw) zx v(wu) xy w(uv) 12(yzzxxy) x(vw) 2 y(wu) 2 0, (2.8) z(uv) 2 which involves three non-negative numbers x, y, z and three strictly positive numbers u, v, w. After some computations we found it is equivalent to m 1 x 2 m 2 y 2 m 3 z 2 n 1 yz n 2 zx n 3 xy uvw(v w)(w u)(u v) [x(v w) 2 y(w u) 2 z(u v) 2 0, (2.9) ] m 1 = u(uv 2vw wu)(v w) 3, m 2 = v(vw 2wu uv)(w u) 3, m 3 = w(wu 2uv vw)(u v) 3, n 1 = (w u)(u v) [ u 2 (v 2 4vw w 2 ) vw(v 2 w 2 ) u(v w)(v 2 10vw w 2 ) ], n 2 = (u v)(v w) [ v 2 (w 2 4wu u 2 ) wu(w 2 u 2 ) v(w u)(w 2 10wu u 2 ) ], n 3 = (v w)(w u) [ w 2 (u 2 4uv v 2 ) Clearly, to prove inequality (2.8) we need to prove that uv(u 2 v 2 ) w(u v)(u 2 10uv v 2 ) ]. m 1 x 2 m 2 y 2 m 3 z 2 n 1 yz n 2 zx n 3 xy 0, (2.10) and hence by replacing x x/(v w), y y/(w u), z z/(u v), it suffices to show that Q r 1 x 2 r 2 y 2 r 3 z 2 s 1 yz s 2 zx s 3 xy 0, (2.11) r 1 = u(v w)(uv 2vw wu), r 2 = v(w u)(vw 2wu uv), r 3 = w(u v)(wu 2uv vw), s 1 = u 2 (v 2 4vw w 2 ) vw(v 2 w 2 ) u(v w)(v 2 10vw w 2 ), s 2 = v 2 (w 2 4wu u 2 ) wu(w 2 u 2 ) v(w u)(w 2 10wu u 2 ), s 3 = w 2 (u 2 4uv v 2 ) uv(u 2 v 2 ) w(u v)(u 2 10uv v 2 ). In the sequel we shall apply the method of the Difference Substitution to prove inequality Q 0. Because of the symmetry in (1.5), without any loss of generality, suppose that a b c. This imply u v w, so we put { v = w p, (p 0) (2.12) u = w p q (q 0). Under these hypotheses, we shall divide our argument into the following six cases: Case 1. The non-negative real numbers x, y, z satisfy x y z. In this case, we set { y = z m, (m 0) x = z m n (n 0). (2.13) By putting (2.12) and (2.13) into the expression of Q, after analyzing we obtain the following identity: k 1 Q a 1 w 4 a 2 w 3 a 3 w 2 a 4 w a 5 w 2 Q 2 1, (2.14)

3 162 J. Liu k 1 = 12(p 2 pq q 2 ), a 1 = (92m 2 92mn 95n 2 )p 2 2q(46m 2 43mn 46n 2 )p q 2 (95m 2 92mn 92n 2 ), a 2 = (208m 2 208mn 268n 2 )p 3 6q(52m 2 56mn 69n 2 )p 2 12q 2 (27m 2 29mn 34n 2 )p 2q 3 (55m 2 76mn 76n 2 ), a 3 = (128m 2 128mn 248n 2 )p 4 8q(32m 2 38mn 65n 2 )p 3 3q 2 (92m 2 116mn 185n 2 )p 2 2q 3 (74m 2 119mn 158n 2 )p 11q 4 (m 2n) 2, a 4 = 24(p 2 qp q 2 )[(4m 2 4mn 4n 2 12zm 6zn 10z 2 )p 3 q(6m 2 6mn 6n 2 18zm 8zn 15z 2 )p 2 q 2 (4m 2 4mn 2n 2 12zm 5zn 9z 2 )p q 3 (m 2z)(m n z)], a 5 = 12p(p q)(p 2 qp q 2 )(qm 2pm pn qn qz 2pz)(2pm qm pn 4pz 2qz), Q 1 = (4m 2n 12z)p 2 ( 2wm 4qm wn qn 12qz)p q(7qm wm 2qn 2wn 12qz). Note that p 0, q 0, m 0, n 0, z 0, hence k 1 0, a i 0, i = 1, 2, 3, 4, 5. Since again w > 0, thus Q 0 follows from the identity (2.14). Case 2. The non-negative real numbers x, y, z satisfy x z y. We set { z = y m, (m 0) (2.15) x = y m n (n 0). By putting (2.12) and (2.15) into the expression of Q, we get k 2 = 12(p 2 pq q 2 ), k 2 Q b 1 w 4 b 2 w 3 b 3 w 2 b 4 w b 5 w 2 Q 2 2, (2.16) b 1 = (95m 2 98mn 95n 2 )p 2 2q(49m 2 49mn 46n 2 )p q 2 (95m 2 92mn 92n 2 ), b 2 = (268m 2 328mn 268n 2 )p 3 6q(65m 2 82mn 69n 2 )p 2 12q 2 (32m 2 39mn 34n 2 )p 2q 3 (55m 2 76mn 76n 2 ), b 3 = (248m 2 368mn 248n 2 )p 4 8q(59m 2 92mn 65n 2 )p 3 3q 2 (161m 2 254mn 185n 2 )p 2 2q 3 (113m 2 197mn 158n 2 )p 11q 4 (m 2n) 2, b 4 = 24(p 2 qp q 2 )[(8m 2 10mn 4n 2 14ym 6yn 10y 2 )p 3 q(11m 2 14mn 6n 2 20ym 8yn 15y 2 )p 2 q 2 (4m 2 5mn 2n 2 11ym 5yn 9y 2 )p q 3 (m 2y)(m n y)], b 5 = 12p(p q)(p 2 qp q 2 )(pm qm pn qn yq 2yp)(3pm qm pn 2yq 4yp) Q 2 = (10m 2n 12y)p 2 (wm 7qm qn wn 12yq)p q(7qm wm 2qn 2wn 12yq). Since p 0, q 0, m 0, n 0, y 0, we have k 2 0, b i 0, i = 1, 2, 3, 4, 5. Therefore, inequality Q 0 follows from (2.16) and w > 0. Case 3. The non-negative real numbers x, y, z satisfy y z x. We put { z = x m, (m 0) (2.17) y = x m n (n 0). By putting relations (2.12) and (2.17) into the expression of Q, it is easy to check the following identity: k 3 Q c 1 w 6 c 2 w 5 c 3 w 4 c 4 w 3 c 5 w 2 c 6 w c 7 w 2 Q 2 3, (2.18)

4 A weighted inequality involving the sides of a triangle 163 k 3 = 21p 2 (22w 20q)p (q 2w)(3q 4w), c 1 = 48m 2, c 2 = 24m(11pm 6qm px qx), c 3 = (623m 2 214xm 95x 2 )p 2 14q(48m 2 19xm 7x 2 )p q 2 (168m 2 144xm 95x 2 ), c 4 = (828m 2 160mn 64n 2 688xm 96xn 428x 2 )p 3 2q(657m 2 128mn 48n 2 546xm 80xn 315x 2 )p 2 4q 2 (159m 2 28mn 8n 2 189xm 28xn 132x 2 )p 2q 3 (36m 2 76xm 8xn 71x 2 ), c 5 = (696m 2 472mn 184n xm 312xn 752x 2 )p 4 4q(357m 2 240mn 90n 2 524xm 162xn 365x 2 )p 3 q 2 (995m 2 652mn 216n xm 556xn 1259x 2 )p 2 2q 3 (114m 2 70mn 20n 2 291xm 96xn 255x 2 )p xq 4 (40m 20n 51x), c 6 = (402m 2 508mn 190n 2 808xm 384xn 596x 2 )p 5 2q(503m 2 633mn 233n xm 481xn 731x 2 )p 4 2q 2 (457m 2 565mn 195n 2 971xm 486xn 709x 2 )p 3 2q 3 (169m 2 203mn 63n 2 445xm 232xn 339x 2 )p 2 2q 4 (18m 2 21mn 6n 2 89xm 46xn 77x 2 )p 6xq 5 (2m n 2x), c 7 = p(p q)(21p 2 20qp 3q 2 )(pm pn qx 2px)(2qm 3pm pn qn 4px 2qx), Q 3 = (17m 21n 2x)p 2 (13wm 20qm 20qn 22wn wx 5qx)p 6q 2 m 4w 2 m wxq 8wqm 5xq 2 10qwn 3q 2 n 8w 2 n. Since w > 0, p 0, q 0, m 0, n 0, x 0, we have k 3 0, c i 0, i = 1, 2, 3, 4, 5, 6, 7. From identity (2.18), we see that inequality Q 0 holds. Case 4. The non-negative real numbers x, y, z satisfy y x z. In this case we put { x = z m, (m 0) y = z m n (n 0). (2.19) By putting (2.12) and (2.19) into the expression of Q, we now have the identity: k 4 Q d 1 w 6 d 2 w 5 d 3 w 4 d 4 w 3 d 5 w 2 d 6 w d 7 w 2 Q 2 4, (2.20) k 4 = 12p 2 (16w 12q)p 8w 2 8wq 5q 2, d 1 = 48n 2,

5 164 J. Liu d 2 = 24n(10pn 5qn zq), d 3 = (504n 2 92z 2 )p 2 4q(126n 2 23nz 23z 2 )p q 2 (119n 2 46nz 95z 2 ), d 4 = (64m 2 64mn 192mz 568n 2 96nz 368z 2 )p 3 4q(24m 2 24mn 72mz 213n 2 74nz 138z 2 )p 2 4q 2 (16m 2 16mn 48mz 102n 2 62nz 117z 2 )p 2q 3 (8m 2 8mn 24mz 31n 2 34nz 71z 2 ), d 5 = (160m 2 160mn 480mz 352n 2 240nz 512z 2 )p 4 8q(40m 2 40mn 120mz 88n 2 73nz 128z 2 )p 3 4q 2 (66m 2 66mn 198mz 125n 2 138nz 221z 2 )p 2 4q 3 (26m 2 26mn 78mz 37n 2 59nz 93z 2 )p q 4 (16m 2 16mn 48mz 11n 2 38nz 43z 2 ), d 6 = (160m 2 160mn 480mz 112n 2 240nz 368z 2 )p 5 8q(50m 2 50mn 150mz 35n 2 76nz 115z 2 )p 4 4q 2 (106m 2 106mn 318mz 66n 2 163nz 241z 2 )p 3 2q 3 (118m 2 118mn 354mz 58n 2 178nz 263z 2 )p 2 2q 4 (36m 2 36mn 108mz 10n 2 51nz 77z 2 )p 10q 5 (m z)(m n 2z), d 7 = (p q)(12p 2 12pq 5q 2 )(qm 2pm pn 2pz qz)(qm 2pm pn qn 4pz 2qz)p, Q 4 = (12m 6n 4z)p 2 (12qm 16wm 8wn 5qn 2wz 4qz)p 8wqm 8w 2 m 5q 2 m 3wqn 2q 2 n 4w 2 n 7q 2 z wqz. Because p 0, q 0, w > 0, m 0, n 0, z 0, we have k 4 0, d i 0, i = 1, 2, 3, 4, 5, 6, 7. Therefore, Q 0 holds true. Case 5. The non-negative real numbers x, y, z satisfy z x y. Put { x = y m, (m 0) z = y m n (n 0). (2.21) By putting (2.12) and(2.21) into the expression of Q and making the calculations, we obtain k 5 Q = e 1 w 6 e 2 w 5 e 3 w 4 e 4 w 3 e 5 w 2 e 6 w e 7 w 2 Q 2 5, (2.22) k 5 = 29p 2 (24w 22q)p 8w 2 8wq 5q 2, e 1 = 48n 2, e 2 = 24(11pn 5qn yp yq)n, e 3 = (95y 2 122yn 623n 2 )p 2 14q(7y 2 12yn 41n 2 )p q 2 (95y 2 46yn 119n 2 ),

6 A weighted inequality involving the sides of a triangle 165 e 4 = (128m 2 224ym 428y 2 384yn 96mn 732n 2 )p 3 2q(88m 2 160ym 64mn 322yn 315y 2 521n 2 )p 2 4q 2 (86yn 111n 2 132y 2 44ym 12mn 16m 2 )p 2q 3 (24ym 34yn 31n 2 8m 2 8mn 71y 2 ), e 5 = (416n 2 792y 2 408m 2 304mn 752ym 640yn)p 4 8q(100n 2 185y 2 65mn 169ym 157yn 89m 2 )p 3 q 2 (503n 2 408m 2 304mn 922yn 1235y 2 960ym)p 2 2q 3 (60m 2 241y 2 172ym 156yn 52mn 65n 2 )p q 4 (16mn 11n 2 48ym 43y 2 16m 2 38yn), e 6 = (396mn 536m 2 560yn 116n 2 772y ym)p 5 2q(432mn 591m 2 879y 2 131n ym 636yn)p 4 2q 2 (361mn 825y 2 570yn 105n ym 486m 2 )p 3 2q 3 (395y 2 253yn 204m 2 163mn 37n 2 521ym)p 2 2q 4 (41mn 56yn 5n 2 46m 2 133ym 97y 2 )p 10q 5 (y m)(2y n m), e 7 = p(p q)(29p 2 22pq 5q 2 )(pm qm 2yp yq)(3pm 2pn qm qn 4yp 2yq), Q 5 = (29m 12n 10y)p 2 (wy 11wn 24mw 22qm 8qn 7yq)p 8mw 2 8mwq 5q 2 m wqy 4w 2 n 3wqn 7q 2 y 2q 2 n). Note that p 0, q 0, w > 0, m 0, n 0, y 0, and k 5 > 0, e i 0, i = 1, 2, 3, 4, 5, 6, 7. Hence Q 0 follows from (2.22). Case 6. The non-negative real numbers x, y, z satisfy z y x. We put { y = x m, (m 0) (2.23) z = x m n (n 0). By putting (2.12) and (2.23) into the expression of Q, we have Q = 8(m 2 mn n 2 )w 4 (24m 2 p 16m 2 q 22mnp 16mnq 20n 2 p 10n 2 q 2xmp 4xmq 4xnp 2xnq)w 3 (29m 2 p 2 36m 2 pq 12m 2 q 2 24mnp 2 32mnpq 12mnq 2 16n 2 p 2 16n 2 pq 3n 2 q 2 20xmp 2 26xmpq 20xmq 2 16xnp 2 16xnpq 10xnq 2 12x 2 p 2 12x 2 pq 12x 2 q 2 )w 2 (16m 2 p 3 26m 2 p 2 q 12m 2 pq 2 12mnp 3 20mnp 2 q 10mnpq 2 4n 2 p 3 6n 2 p 2 q 2n 2 pq 2 28xmp 3 44xmp 2 q 26xmpq 2 4xmq 3 16xnp 3 24xnp 2 q 12xnpq 2 2xnq 3 20x 2 p 3 30x 2 p 2 q 18x 2 pq 2 4x 2 q 3 )w (p q)(mp 2xp xq)(3mp 2mq 2np nq 4xp 2xq)p. (2.24) Obviously, inequality Q 0 holds in this case, too. Combining the arguments in the above six cases, we deduce that inequality Q 0 holds for non-negative real numbers x, y, z and strictly positive real numbers u, v, w. Therefore, inequalities (2.8) and (1.5) are proved. We find out that equality in (2.8) holds if and only if x = y = z and u = v = w. Thus the proof of the theorem is complete. 3. AN APPLICATION In this section, we use our Theorem 1.1 to establish a new geometric inequality. Corollary 3.1. Let P be an interior point of triangle ABC and let D, E, F be the feet of the perpendiculars from P to the sides BC, CA, AB, respectively. Denote the altitudes of pedal triangle DEF by h d, h e, h f. Then r a, r b, r c are the radii of corresponding three excircles of triangle ABC. r a h d r b h e r c h f 6, (3.25) Proof. In what follows, we let R, denote the circumradius and the area of triangle ABC, respectively; p denotes the area of the pedal triangle DEF. We also set P D = r 1, P E = r 2, P F = r 3, P A = R 1, P B = R 2, P C = R 3. In the

7 166 J. Liu Theorem 1.1 we take x = r 1 a, y = r 2 b, z = r 3 c, then ( 1 cr2 br 3 abc s a ar 3cr 1 br ) 1ar 2 s b s c Note that by the well known relation abc = 4R and the following identities: 12 ( r2 r 3 ar 1 br 2 cr 3 bc r 3r 1 ca r ) 1r 2. ab ar 1 br 2 cr 3 = 2, (3.26) r 2 r 3 bc r 3r 1 ca r 1r 2 ab = p, (3.27) we get cr 2 br 3 ar 3 cr 1 br 1 ar 2 24 R p s a s b s c, and then by using the fact (s a)r a = (s b)r b = (s c)r c = and the well-known inequality (see [3]): etc., we have Note that again EF = ar 1 2R, F D = br 2 2R, DE = cr 3 2R, h d = and so the claimed inequality follows from (3.29) immediately. ar 1 cr 2 br 3, (3.28) ar 1 r a br 2 r b cr 3 r c 24R p. (3.29) 4. TWO CONJECTURES p 2EF, h e = p 2F D, h f = p 2DE, In the last section, we propose two conjectures which have been checked by the computer. Conjecture 4.1. For any triangle ABC and arbitrary positive real numbers x, y, z, the following inequality holds: (s b)(s c) y z (s c)(s a) z x (s a)(s b) x y 3(xa2 yb 2 zc 2 ) 4(yz zx xy). (4.30) If the above inequality holds, then by applying the same procedure as in the proof of corollary, we can get the geometric inequality: h d h e h f 3 r b r c r c r a r a r b 4. (4.31) But the inequality is still not proved. On the other hand, the author thinks the following stronger inequality holds: Conjecture 4.2. Let P be an interior point of triangle ABC, then h d w a h e w b h f w c 3 2, (4.32) w a, w b, w c are the corresponding angle-bisectors of triangle ABC. REFERENCES [1] Wolstenholme, J., A Book of Mathematical Problems on Subjects Included in the Cambridge Course, London and Cambridge, 1867 [2] Mitrinović, D., Pečarić, J. E. and Volenec, V., Recent Advances in Geometric Inequalities, Kluwer Academic Publishers, Dordrecht/Boston London, 1989 [3] Bottema, O., Djordjević, R. Ž., Janić, R. R., Mitrinović, D. S. and Vasić, P. M., Geometric Inequalities, Groningen, 1969 [4] Liu, J., An inequality involvign sides of a triangle and its applications, Zhong Xue Shu Xue 11 (1992), (in Chinese) [5] Liu, Jian, Some new inequalities for the triangle, Zhong Xue Shu Xue 5 (1994), 9-12 (in Chinese) [6] Liu, Jian, A new weighted inequality related to the sides of an acute triangle, J. of Foshan Univ. (Nat. Sci. Ed.), 21 (2003), No. 4, 4-7 (in Chinese) [7] Liu, Jian, The Composite Theorem of Ternary Quadratic Inequalities and its Applications, RGMIA Research Report Collection 11 (2008), No. 4, http: //www. staff. vu. edu. au/rgmia/v11n4. asp [8] Yu-Dong Wu, A new proof of a weighted Erdös-Mordell type inequality, Forum Geometricorum 8 (2008), , http: //forumgeom. fau. edu/fg2008volume8/index. html [9] Yu-Dong Wu, Zhi-hua Zhang and Yu-Rui Zhang, Proving inequalities in acute triangle with difference substitution, Inequal. Pure Appl. Math. 8 (2007), No. 3, Art. 81 [10] Yang, Lu Difference substitution and automated inequality proving, J. Guangzhou Univ. Nat. Sci. 5 (2006), No. 2, 1-7 (in Chinese) EAST CHINA JIAOTONG UNIVERSITY, NANCHANG CITY, JIANGXI PROVINCE, , P. R. CHINA address:

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