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#1
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¢Í¤ÇÒÁªèÇÂàËÅ×ÍàÃ×èͧµÃÕ⡳ÍÕ¡¤ÃÑ駤ÃѺ¼Á
1.¨§ËÒ¤èÒ x â´Â $x\epsilon [0,2\pi ]$
$2cos^2(x/2)sin^2(x/2) = x^2 + (1/x^2)$ 2. ¡Ó˹´ÃÙ»ÊÒÁàËÅÕèÂÁ ABC ãËé a,b,c à»ç¹´éÒ¹µÃ§¢éÒÁÁØÁ A,B áÅÐ C µÒÁÅӴѺ ¶éÒ $ a^4 + b^4 + c^4 = 2c^2(a^2 + b^2)$ ¨§ËÒ ÁØÁ C ¢Í¤ÇÒÁ¡ÃسÒÍÕ¡¤ÃÑ駹ФÃѺ ¢Íº¤Ø³ÁÒ¡¤ÃѺ¼Á |
#2
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ÍéÒ§ÍÔ§:
L.H.S = $(1/2)(4\sin^2(x/2)\cos^2(x/2)) = (1/2)(\sin^2x))$ ´Ñ§¹Ñé¹ $ 0 \le L.H.S \le 1/2$ áµè R.H.S = $(x-1/x)^2 + 2 \ge 2$ ¨Ö§à»ç¹ä»äÁèä´é·Õè L.H.S = R.H.S. ÊÁ¡Òè֧äÁèÁդӵͺ ¢éÍ 2. ¨Ò¡¡®¢Í§â¤ä«¹ì $c^2 = a^2 + b^2 - 2ab\cos C$ ¨Ðä´é $a^2+b^2=(c^2+2ab\cos C)$ ...(*) ¨Ñ´Ãٻ⨷ÂìãËÁèà»ç¹ $(a^2+b^2)^2-2a^2b^2+c^4 = 2c^2(a^2+b^2)$ á×·¹¤èÒ¨Ò¡ÊÁ¡Òà (*) Å§ä» ¨Ðä´é $\cos^2C = 1/2$ ´Ñ§¹Ñé¹ C = $\pi/4, 3\pi/4$ àÁ×è͵ÃǨ¤ÓµÍº ¨Ð¾ºÇèÒà»ç¹¨ÃÔ§·Ñé§Êͧ¤èÒ |
#3
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¢Íº¤Ø³ÁÒ¡¤ÃѺ¼Á
¢ÍÍÀÑ·ÕèÁҢͺ¤Ø³ªéҹФÃѺ ªèǧ¹ÕéÂØè§ÁÒ¡ äÁè¤èÍÂä´éà¢éÒàǺàÅÂÍФÃѺ |
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