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Nội dung: <blockquote data-quote="NguoiDien" data-source="post: 162120" data-attributes="member: 75">Bài III1. Đặt <img src="https://onthitoan.com/wp-content/ql-cache/quicklatex.com-cf5da0a080b1557a1589e2a54c811089_l3.png" alt="" class="fr-fic fr-dii fr-draggable " data-size="" style="" /> và <img src="https://onthitoan.com/wp-content/ql-cache/quicklatex.com-f0e7d767de7d89c63f92dd116657b231_l3.png" alt="" class="fr-fic fr-dii fr-draggable " data-size="" style="" /> ta có hệ: <img src="https://onthitoan.com/wp-content/ql-cache/quicklatex.com-e076102e070b04e959e11066661e4ce4_l3.png" alt="" class="fr-fic fr-dii fr-draggable " data-size="" style="" /> <img src="https://onthitoan.com/wp-content/ql-cache/quicklatex.com-fe679bf33335aab881eb1b4f931ded91_l3.png" alt="" class="fr-fic fr-dii fr-draggable " data-size="" style="" />. Cộng hai phương trình ta có <img src="https://onthitoan.com/wp-content/ql-cache/quicklatex.com-cb368103c10ad7ab845081decdafe852_l3.png" alt="" class="fr-fic fr-dii fr-draggable " data-size="" style="" />. thế vào phương trình thứ hai ta có <img src="https://onthitoan.com/wp-content/ql-cache/quicklatex.com-dc115e08b6bf6051c3b9dca952fed4aa_l3.png" alt="" class="fr-fic fr-dii fr-draggable " data-size="" style="" /> Do đó <img src="https://onthitoan.com/wp-content/ql-cache/quicklatex.com-eb35bc4d69ad0900b2fb77658b4db2a6_l3.png" alt="" class="fr-fic fr-dii fr-draggable " data-size="" style="" />. Thế <img src="https://onthitoan.com/wp-content/ql-cache/quicklatex.com-cb368103c10ad7ab845081decdafe852_l3.png" alt="" class="fr-fic fr-dii fr-draggable " data-size="" style="" /> và <img src="https://onthitoan.com/wp-content/ql-cache/quicklatex.com-c93ec615a42832af3b3eab96349604c5_l3.png" alt="" class="fr-fic fr-dii fr-draggable " data-size="" style="" /> vào <img src="https://onthitoan.com/wp-content/ql-cache/quicklatex.com-cf5da0a080b1557a1589e2a54c811089_l3.png" alt="" class="fr-fic fr-dii fr-draggable " data-size="" style="" /> ta được <img src="https://onthitoan.com/wp-content/ql-cache/quicklatex.com-0268fc45ac3d9d0e4508d7c5a5d72c4b_l3.png" alt="" class="fr-fic fr-dii fr-draggable " data-size="" style="" />. Vậy nghiệm của hệ là <img src="https://onthitoan.com/wp-content/ql-cache/quicklatex.com-a2a4ba687fe13f12fa0954521cd08677_l3.png" alt="" class="fr-fic fr-dii fr-draggable " data-size="" style="" /> 2. Hoành độ giao điểm của <img src="https://onthitoan.com/wp-content/ql-cache/quicklatex.com-dea768d63c4c9c182714f1c0d5d3a6b2_l3.png" alt="" class="fr-fic fr-dii fr-draggable " data-size="" style="" /> và <img src="https://onthitoan.com/wp-content/ql-cache/quicklatex.com-265493801b051f380ab13b5035ecfa7e_l3.png" alt="" class="fr-fic fr-dii fr-draggable " data-size="" style="" /> là nghiệm phương trình <img src="https://onthitoan.com/wp-content/ql-cache/quicklatex.com-aa8157e9a82b3d5b3247600063e190af_l3.png" alt="" class="fr-fic fr-dii fr-draggable " data-size="" style="" />. Giải phương trình ta được hai nghiệm <img src="https://onthitoan.com/wp-content/ql-cache/quicklatex.com-c4b4771b268013cfec859da66df0186b_l3.png" alt="" class="fr-fic fr-dii fr-draggable " data-size="" style="" /> và <img src="https://onthitoan.com/wp-content/ql-cache/quicklatex.com-d74f2692634bcc5ab43f63857c8546e2_l3.png" alt="" class="fr-fic fr-dii fr-draggable " data-size="" style="" />. Khi đó ta được các giao điểm của <img src="https://onthitoan.com/wp-content/ql-cache/quicklatex.com-dea768d63c4c9c182714f1c0d5d3a6b2_l3.png" alt="" class="fr-fic fr-dii fr-draggable " data-size="" style="" /> và <img src="https://onthitoan.com/wp-content/ql-cache/quicklatex.com-265493801b051f380ab13b5035ecfa7e_l3.png" alt="" class="fr-fic fr-dii fr-draggable " data-size="" style="" /> là <img src="https://onthitoan.com/wp-content/ql-cache/quicklatex.com-5ac8d711ff75e8cd2bd207fde87d0d24_l3.png" alt="" class="fr-fic fr-dii fr-draggable " data-size="" style="" /> và <img src="https://onthitoan.com/wp-content/ql-cache/quicklatex.com-af6e5efe1513a0a68c2afee17f5ad445_l3.png" alt="" class="fr-fic fr-dii fr-draggable " data-size="" style="" />. Đường thẳng <img src="https://onthitoan.com/wp-content/ql-cache/quicklatex.com-f71e590bc2c612d45c114f2396efa86c_l3.png" alt="" class="fr-fic fr-dii fr-draggable " data-size="" style="" /> có phương trình <img src="https://onthitoan.com/wp-content/ql-cache/quicklatex.com-defc811302fe145258c58d10f8c48e3f_l3.png" alt="" class="fr-fic fr-dii fr-draggable " data-size="" style="" />. Đường thẳng qua <img src="https://onthitoan.com/wp-content/ql-cache/quicklatex.com-bb37b78c388ce0097f18af4b756bded8_l3.png" alt="" class="fr-fic fr-dii fr-draggable " data-size="" style="" /> và vuông góc với <img src="https://onthitoan.com/wp-content/ql-cache/quicklatex.com-f71e590bc2c612d45c114f2396efa86c_l3.png" alt="" class="fr-fic fr-dii fr-draggable " data-size="" style="" /> có hệ số góc <img src="https://onthitoan.com/wp-content/ql-cache/quicklatex.com-b3c3c577d92eba27de786d8cad8cbda3_l3.png" alt="" class="fr-fic fr-dii fr-draggable " data-size="" style="" />. Do đường thẳng đó qua <img src="https://onthitoan.com/wp-content/ql-cache/quicklatex.com-bb37b78c388ce0097f18af4b756bded8_l3.png" alt="" class="fr-fic fr-dii fr-draggable " data-size="" style="" /> nên ta có phương trình đường thẳng qua <img src="https://onthitoan.com/wp-content/ql-cache/quicklatex.com-bb37b78c388ce0097f18af4b756bded8_l3.png" alt="" class="fr-fic fr-dii fr-draggable " data-size="" style="" /> và vuông góc với <img src="https://onthitoan.com/wp-content/ql-cache/quicklatex.com-f71e590bc2c612d45c114f2396efa86c_l3.png" alt="" class="fr-fic fr-dii fr-draggable " data-size="" style="" /> là <img src="https://onthitoan.com/wp-content/ql-cache/quicklatex.com-f53c4de75153d5f4baa52a311e29bf2d_l3.png" alt="" class="fr-fic fr-dii fr-draggable " data-size="" style="" />. Khi đó chân đường cao của tam giác <img src="https://onthitoan.com/wp-content/ql-cache/quicklatex.com-6504f6733bba9b828174605bde93ead8_l3.png" alt="" class="fr-fic fr-dii fr-draggable " data-size="" style="" /> kẻ từ <img src="https://onthitoan.com/wp-content/ql-cache/quicklatex.com-bb37b78c388ce0097f18af4b756bded8_l3.png" alt="" class="fr-fic fr-dii fr-draggable " data-size="" style="" /> là nghiệm của hệ: <img src="https://onthitoan.com/wp-content/ql-cache/quicklatex.com-78ec8e83807ffbd72ba503d9cdbfaabe_l3.png" alt="" class="fr-fic fr-dii fr-draggable " data-size="" style="" /> Giải hệ trên ta được <img src="https://onthitoan.com/wp-content/ql-cache/quicklatex.com-1e0e0cf627fe0a758e7435b78fde4642_l3.png" alt="" class="fr-fic fr-dii fr-draggable " data-size="" style="" />. Như vậy chân đường cao <img src="https://onthitoan.com/wp-content/ql-cache/quicklatex.com-04a0810783ccf74e5d2622c1163edddc_l3.png" alt="" class="fr-fic fr-dii fr-draggable " data-size="" style="" />. Độ dài cạnh đáy <img src="https://onthitoan.com/wp-content/ql-cache/quicklatex.com-6b73e82ed0db12365a3b23b5d855b09d_l3.png" alt="" class="fr-fic fr-dii fr-draggable " data-size="" style="" /> Độ dài đường cao <img src="https://onthitoan.com/wp-content/ql-cache/quicklatex.com-fb57c3542a7f822dd1e905ee0773b3dd_l3.png" alt="" class="fr-fic fr-dii fr-draggable " data-size="" style="" /> Suy ra diện tích tam giác <img src="https://onthitoan.com/wp-content/ql-cache/quicklatex.com-6504f6733bba9b828174605bde93ead8_l3.png" alt="" class="fr-fic fr-dii fr-draggable " data-size="" style="" />: <img src="https://onthitoan.com/wp-content/ql-cache/quicklatex.com-7ce6daa485fc349d37f9bc8afc94a07f_l3.png" alt="" class="fr-fic fr-dii fr-draggable " data-size="" style="" />(đvdt).</blockquote>

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