{"id":3766,"date":"2021-08-08T07:26:00","date_gmt":"2021-08-08T07:26:00","guid":{"rendered":"https:\/\/mathemerize.com\/?p=3766"},"modified":"2021-11-30T19:32:30","modified_gmt":"2021-11-30T14:02:30","slug":"conditional-trigonometric-identities","status":"publish","type":"post","link":"https:\/\/mathemerize.com\/conditional-trigonometric-identities\/","title":{"rendered":"Conditional Trigonometric Identities – Maximum & Minimum Value"},"content":{"rendered":"
Here, you will learn conditional trigonometric identities and maximum and minimum value in trigonometry.<\/p>\n
Let’s begin –<\/p>\n
(i)\u00a0 acos\\(\\theta\\) + bcos\\(\\theta\\) will always lie in the interval [-\\(\\sqrt{a^2+b^2}\\), \\(\\sqrt{a^2+b^2}\\)] i.e. the maximum and minimum values are \\(\\sqrt{a^2+b^2}\\), -\\(\\sqrt{a^2+b^2}\\) respectively.<\/p>\n
(ii)\u00a0 Minimum value of \\(a^2tan^2\\theta\\) + \\(b^2\\tan^2\\theta\\) = 2ab where a,b > 0<\/p>\n
(iii)\u00a0 -\\(\\sqrt{a^2 + b^2 + 2abcos(\\alpha – \\beta)}\\) \\(\\le\\) acos(\\(\\alpha + \\theta\\)) + bcos(\\(\\beta + \\theta\\)) \\(\\le\\) \\(\\sqrt{a^2 + b^2 + 2abcos(\\alpha – \\beta)}\\) where \\(\\alpha\\) and \\(\\beta\\) are known angles.<\/p>\n
(iv)\u00a0 In case a quadratic in sin\\(\\theta\\) & cos\\(\\theta\\) is given then the maximum and minimum values can be obtained by making perfect square.<\/p>\n\n\n
Example : <\/span> Find the maximum value of 1 + \\(sin({\\pi\\over 4} + \\theta)\\) + 2\\(cos({\\pi\\over 4} — \\theta)\\)<\/p>\n Solution : <\/span>We have 1 + \\(sin({\\pi\\over 4} + \\theta)\\) + 2\\(cos({\\pi\\over 4} — \\theta)\\) If A + B + C = \\(180^{\\circ}\\),then<\/p>\n (i) tanA + tanB + tanC = tanA tanB tanC<\/p>\n (ii) cotA cotB + cotB cotC + cotC cotA = 1<\/p>\n (iii) \\(tan{A\\over 2}\\) \\(tan{B\\over 2}\\) + \\(tan{B\\over 2}\\) \\(tan{C\\over 2}\\) + \\(tan{C\\over 2}\\) \\(tan{A\\over 2}\\) = 1<\/p>\n (iv) \\(cot{A\\over 2}\\) + \\(cot{B\\over 2}\\) + \\(cot{C\\over 2}\\) = \\(cot{A\\over 2}\\) \\(cot{B\\over 2}\\) \\(cot{C\\over 2}\\)<\/p>\n (v) sin2A + sin2B + sin2C = 4sinA sinB sinC<\/p>\n (vi) cos2A + cos2B + cos2C = 1 – 4cosA cosB cosC<\/p>\n (vii) sinA + sinB + sinC = 4\\(cos{A\\over 2}\\) \\(cos{B\\over 2}\\) \\(cos{C\\over 2}\\)<\/p>\n (viii) cosA + cosB + cosC = 1 + 4\\(sin{A\\over 2}\\) \\(sin{B\\over 2}\\) \\(sin{C\\over 2}\\)<\/p>\n Some Important results :<\/strong><\/p>\n (i) sinA sin(\\(60^{\\circ}\\) – A) sin(\\(60^{\\circ}\\) + A) = \\(1\\over 4\\)sin3A<\/p>\n (ii) cosA cos(\\(60^{\\circ}\\) – A) cos(\\(60^{\\circ}\\) + A) = \\(1\\over 4\\)cos3A<\/p>\n (iii) tanA tan(\\(60^{\\circ}\\) – A) tan(\\(60^{\\circ}\\) + A) = tan3A<\/p>\n (iv) cotA cot(\\(60^{\\circ}\\) – A) cot(\\(60^{\\circ}\\) + A) = cot3A<\/p>\n (v) \\(sin^2A\\) + \\(sin^2(60^{\\circ}\\) – A) + \\(sin^2(60^{\\circ}\\) + A) = \\(3\\over 2\\)<\/p>\n (vi) \\(cos^2A\\) + \\(cos^2(60^{\\circ}\\) – A) + \\(cos^2(60^{\\circ}\\) + A) = \\(3\\over 2\\)<\/p>\n (vii) tanA + tan(\\(60^{\\circ}\\) + A) + tan(\\(120^{\\circ}\\) + A) = 3tan3A<\/p>\n\n\n
\n = 1 + \\(1\\over sqrt{2}\\)(cos\\(\\theta\\) + sin\\(\\theta\\)) + \\(\\sqrt{2}\\)(cos\\(\\theta\\) + sin\\(\\theta\\)) = 1 + (\\({1\\over \\sqrt{2}} + \\sqrt{2}\\)) (cos\\(\\theta\\) + sin\\(\\theta\\))
\n = 1 + (\\({1\\over \\sqrt{2}} + \\sqrt{2}\\)) . \\(\\sqrt{2}\\) = 4
<\/p>\n\n\nConditional Trigonometric Identities :<\/h2>\n