Cho hình thang ABCD có đáy lớn AD gấp 3 lần đáy nhỏ BC. Tính diện tích hình thang biết diện tích hình tam giác BCD là 254cm2.
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Gọi đáy lớn ad là a, đáy nhỏ bc là b, đường cao là h
Ta có: S tam giác BCD là: (b x h) : 2= 254 (cm2) => b x h = 254 x 2=508(cm) (1)
Có: S hình thang ABCD= ((a+b)x h):2=> (a x h+b x h): 2
mà a=3xb
=>(3 x b x h+b x h):2 (2)
Từ (1) và (2) ta có: (3 x 508 + 508):2= S hình thang abcd
=> 2032 : 2 =S hình thang ABCD
=>S hình thang ABCD= 1016(cm2)
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