Tài liệu đề thi học sinh giỏi toán lớp 8

Tröôøng THCS Ngoâ Gia Töï. Lôùp:…… Teân:………………………………………….. ÑIEÅM SOÁ Ñieåm baèng chöõ ÑEÀ THI CHOÏN HOÏC SINH GIOÛI KHOÁI 8 MOÂN: TOAÙN ( Thôøi gian 90 phuùt ) Chöõ kyù giaùm khaûo Chöõ kyù giaùm thò Baøi 1: Giaûi caùc phöông trình vaø caùc baát phöông trình sau : ( 4 ñ ) x2 x4 x6 x8    a) 98 96 94 92 ........................................................................................................................................................................ ........................................................................................................................................................................ ........................................................................................................................................................................ ........................................................................................................................................................................ ........................................................................................................................................................................ ........................................................................................................................................................................ ........................................................................................................................................................................ ........................................................................................................................................................................ x 1 1 b) x3 ........................................................................................................................................................................ ........................................................................................................................................................................ ........................................................................................................................................................................ ........................................................................................................................................................................ ........................................................................................................................................................................ ........................................................................................................................................................................ ........................................................................................................................................................................ x3 x2  2 c) 3 2 ........................................................................................................................................................................ ........................................................................................................................................................................ ........................................................................................................................................................................ ........................................................................................................................................................................ ........................................................................................................................................................................ ........................................................................................................................................................................ ........................................................................................................................................................................ d) x  2  3  1 ........................................................................................................................................................................ ........................................................................................................................................................................ 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Baøi 2: Ruùt goïn bieåu thöùc roài tìm giaù trò cuûa bieán ñeå bieåu thöùc: (2ñ) x 2  4x  4 coù giaù trò döông x 3  6x 2  12x  8 ........................................................................................................................................................................ ........................................................................................................................................................................ ........................................................................................................................................................................ ........................................................................................................................................................................ ........................................................................................................................................................................ ........................................................................................................................................................................ ........................................................................................................................................................................ ........................................................................................................................................................................ ........................................................................................................................................................................ ........................................................................................................................................................................  x x3  8 x 2  2x  4  4 B   . b) coù giaù trò döông  : 3 x2  4  x  2 x2 x 8 a) A  ........................................................................................................................................................................ ........................................................................................................................................................................ ........................................................................................................................................................................ ........................................................................................................................................................................ ........................................................................................................................................................................ ........................................................................................................................................................................ ........................................................................................................................................................................ ........................................................................................................................................................................ ........................................................................................................................................................................ ........................................................................................................................................................................ ........................................................................................................................................................................ Baøi 3: Cho tam giaùc ABC coù caùc ñöôøng trung tuyeán AD vaø BE thoûa maõn ñieàu kieän � � CAD  CBE  30o . Chöùng minh ABC laø tam giaùc ñeàu. ........................................................................................................................................................................ ........................................................................................................................................................................ ........................................................................................................................................................................ ........................................................................................................................................................................ ........................................................................................................................................................................ ........................................................................................................................................................................ ........................................................................................................................................................................ ........................................................................................................................................................................ ........................................................................................................................................................................ ........................................................................................................................................................................ ........................................................................................................................................................................ ........................................................................................................................................................................ ........................................................................................................................................................................ ........................................................................................................................................................................ ........................................................................................................................................................................ ........................................................................................................................................................................ ........................................................................................................................................................................ ........................................................................................................................................................................ ........................................................................................................................................................................ ........................................................................................................................................................................ ........................................................................................................................................................................