[ COMBO ] BỒI DƯỠNG TOÁN 8 NÂNG CAO VÀ PHÁT TRIỂN (VŨ HỮU BÌNH-NXBGD) & TUYỂN TẬP ĐỀ THI HSG TOÁN 8 (NGUYỄN VĂN TÚ-THCS THANH MỸ)
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Nơi bồi dưỡng kiến thức Toán - Lý - Hóa cho học sinh cấp 2+3 /<br />
Diễn Đàn Toán - Lý - Hóa 1000B Trần Hưng Đạo Tp.Quy Nhơn Tỉnh Bình Định<br />
Tuyển tập đề thi <strong>HSG</strong> Toán 8<br />
⎧x<br />
− 1 = 2 ⎧x<br />
= 3<br />
⎨ ⇔ ⎨<br />
⎩y<br />
+ 1 = 1 ⎩y<br />
= 0<br />
⎧x<br />
− 1 = − 2 ⎧x<br />
= −1<br />
⎨ ⇔ ⎨ (lo¹i)<br />
⎩y<br />
+ 1 = − 1 ⎩y<br />
= −2<br />
VËy víi (x;y) = (3;0) vµ (x;y) = (0;-3) th× P = 3.<br />
Bµi 2.(2 ®iÓm) §iÒu kiÖn x¸c ®Þnh:<br />
Ta cã :<br />
⎧x<br />
≠ 2<br />
⎪<br />
x ≠ 3<br />
⎪<br />
⎨x<br />
≠ 4<br />
⎪x<br />
≠ 5<br />
⎪<br />
⎪ ⎩x<br />
≠ 6<br />
( )( )<br />
( )( )<br />
( )( )<br />
( )( )<br />
− 5 + 6 = − 2 − 3<br />
2<br />
x x x x<br />
− 7 + 12 = − 3 − 4<br />
2<br />
x x x x<br />
− 9 + 20 = − 4 − 5<br />
2<br />
x x x x<br />
− 11 + 30 = − 5 − 6<br />
2<br />
x x x x<br />
Ph−¬ng tr×nh ®· cho t−¬ng ®−¬ng víi :<br />
1 1 1 1 1<br />
+ + + =<br />
x − 2 x − 3 x − 3 x − 4 x − 4 x − 5 x − 5 x − 6 8<br />
( )( ) ( )( ) ( )( ) ( )( )<br />
1 1 1 1 1 1 1 1 1<br />
⇔ − + − + − + − =<br />
x − 3 x − 2 x − 4 x − 3 x − 5 x − 4 x − 6 x − 5 8<br />
⇔ 1 1 1<br />
x 6 − x 2 = 4 1<br />
8<br />
⇔ x − 6 x − 2 = 8<br />
− − ( )( )<br />
( )( )<br />
⇔ − 8 − 20 = 0 ⇔ − 10 + 2 = 0<br />
2<br />
x x x x<br />
⎡x<br />
= 10<br />
⇔ ⎢ tho m·n ®iÒu kiÖn ph−¬ng tr×nh.<br />
⎣x<br />
= −2<br />
Ph−¬ng tr×nh cã nghiÖm : x = 10; x = -2.<br />
Bµi 3.(2®iÓm)<br />
2<br />
( x + 2) − ( x −1) ( x −1)<br />
2 2<br />
( )<br />
2 2<br />
2 2<br />
2x + 1+ x + 2 − x − 2 x + 2 − x − 2x<br />
+ 1<br />
M = =<br />
2 2<br />
x + 2 x + 2<br />
M = = 1−<br />
2 2<br />
x + 2 x + 2<br />
M lín nhÊt khi ( x −1<br />
)2<br />
2<br />
x + 2<br />
nhá nhÊt.<br />
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V× ( ) 2<br />
2<br />
x −1 ≥ 0∀ x vµ ( x + 2)<br />
〉 0∀ x nªn ( x −1<br />
)2<br />
nhá nhÊt khi<br />
2<br />
( x − 1) 2<br />
= 0.<br />
x + 2<br />
DÊu “=” xy ra khi x-1 = 0 ⇔ x = 1. VËy M max = 1 khi x = 1.<br />
Bµi 4. . (3iÓm)<br />
Gv: Nguyễn Văn Tú<br />
18<br />
Trường <strong>THCS</strong> Thanh Mỹ<br />
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