Quỳnh - Hình học 10

By Trang Ánh Nam -
Giải:
Câu 21:
=
=
=0
Câu 24:
Ta có:
ó  (bài này nghỉ được tới đây mới làm được)
Vẽ đường cao h nữa là xong. (đường cao ứng với AB)
h=S/AB=54/8=27/4
122=(27/4)2 +a2 (a ứng với cãnh kề khi vẽ tam giác)
=>
ð 
Giải câu 75: xem hình không rỏ A(-3,-2), B(3;6), C(11,0) nếu sai tọa độ thì tự sửa
(6;8), =(14,2), =(8,-6)
ta thấy
ü AB=BC
ü  =0 =>AB⊥BC
Vậy để tứ giác ABCD là vuông chỉ cần thêm điều kiện ABCD là hình bình hành.
=>
Làm trắc nghiệm thì không cần trình bài như trên. Chỉ cần lập .
Tới đây tự giải.
ở đây lưu ý nếu điều kiện chuẩn là dùng đường chéo, hai đường chéo cắt nhau tại trung điểm, bằng nhau và vuông góc với nhau nên ta gọi M à trung điểm của AC thì ta sẽ có hệ:
nhưng làm trắc nghiệm thì đi lối tắc ở trên hay hơn).

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