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1．分清“腰、底”例1 已知一个等腰三角形的一边长为5,另一边长为7,则这个等腰三角形的周长是( ) (A)12． (B)17． (C)19． (D)17或19．分析题中并未说明5是底边,还是腰,应分两种情况讨论．解当等腰三角形的一腰长为5时, 7-5<5<7+5, 此时,满足题意的三角形的周长为 7+5+5=17; 当等腰三角形的一腰长为7时, 7-5<7<7+5, 此时,满足题意的三角形的周长为 7+7+5=19．综上知,选(D)．
1. Distinguish between “waist” and “bottom” Example 1 Given that an isosceles triangle has a length of 5 on one side and a length of 7 on the other side, the circumference of this isosceles triangle is () (A) 12. (B)17. (C) 19. (D)17 or 19. The analysis question does not indicate whether 5 is the bottom edge or the waist. It should be discussed in two situations. Solution When the isosceles triangle has a waist length of 5, 7-5<5<7+5, at this time, the circumference of the triangle satisfying the meaning of the question is 7+5+5=17; when the waist is an isosceles triangle When the length is 7, 7-5<7<7+5. At this time, the length of the triangle that satisfies the meaning of the question is 7+7+5=19. To sum up, choose (D).