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Bộ 23 đề thi thử THPTQG năm 2018 - Môn Toán - Gv Đặng Việt Đông - Lovebook - Có lời giải chi tiết

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2 2 2<br />

x y z<br />

Dấu “=” xảy ra khi = = .<br />

2 2 2<br />

a b c<br />

Câu 50: Đáp án B<br />

Gọi<br />

<br />

<br />

<br />

<br />

<br />

( ) Ox = A( a;0;0)<br />

( ) Oy B( b )<br />

( ) Oz = C ( 0;0; c)<br />

x y z<br />

= 0; ;0 ( )<br />

: + + = 1<br />

a b c<br />

Mặt cầu ( S ) có tâm ( 0;0;0 )<br />

I = , bán kính R = 3<br />

Do ( ) tiếp xúc với ( S ) nên d I,<br />

( )<br />

<br />

= R<br />

−1 1 1 1 1<br />

= 3 + + =<br />

2 2 2<br />

1 1 1 a b c 3<br />

+ +<br />

2 2 2<br />

a b c<br />

1 1 1 1 1 1 1<br />

Suy ra T = + + = + + = .<br />

2 2 2 2 2<br />

OA OB OC a b c 3<br />

x y z<br />

+ + − = .<br />

a b c<br />

hay ( ) : 1 0

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