{"common":{"save":0,"post_id":"7056","level":3,"total":10,"point":10,"point_extra":0},"segment":[{"id":"4958","post_id":"7056","mon_id":"1158924","chapter_id":"1159147","question":"<p>Cho c\u1ea5p s\u1ed1 nhân <span class=\"math-tex\">$(u_n)$<\/span> bi\u1ebft <span class=\"math-tex\">$u_1=-2;u_2=-4$<\/span>, khi \u0111ó <span class=\"math-tex\">$u_3$<\/span> b\u1eb1ng<\/p>","options":["<strong>A.<\/strong> 8","<strong>B.<\/strong> 2","<strong>C.<\/strong> –6","<strong>D.<\/strong> –8"],"correct":"4","level":"3","hint":"","answer":"<p>Ta có công b\u1ed9i <span class=\"math-tex\">$q=\\dfrac{u_2}{u_1}=2$<\/span> ⇒ <span class=\"math-tex\">$u_3=u_2.q=-4.2=-8$<\/span> .<\/p><p>Ch\u1ecdn <span style=\"color:#16a085;\"> <strong>D.<\/strong> –8.<\/span><\/p>","type":"choose","extra_type":"classic","user_id":"131","test":"0","date":"2023-12-14 01:14:37","option_type":"txt","len":1},{"id":"4960","post_id":"7056","mon_id":"1158924","chapter_id":"1159147","question":"<p>Cho c\u1ea5p s\u1ed1 c\u1ed9ng <span class=\"math-tex\">$(u_n)$<\/span> có công sai <span class=\"math-tex\">$d=-2$<\/span> ; <span class=\"math-tex\">$S_8=72$<\/span> . Tính <span class=\"math-tex\">$u_1$<\/span> .<\/p>","options":["<strong>A.<\/strong> 16","<strong>B.<\/strong> –16","<strong>C.<\/strong> <span class=\"math-tex\">$\\dfrac{1}{16}$<\/span>","<strong>D.<\/strong> <span class=\"math-tex\">$-\\dfrac{1}{16}$<\/span>"],"correct":"1","level":"3","hint":"","answer":"<p><span class=\"math-tex\">$S_8=8.u_1+\\dfrac{8.7}{2}.d$<\/span> <\/p><p>⇔ <span class=\"math-tex\">$72=8.u_1+28.(-2)$<\/span><\/p><p>⇔ <span class=\"math-tex\">$u_1=16$<\/span><\/p><p>Ch\u1ecdn <span style=\"color:#16a085;\"> <strong>A.<\/strong> 16.<\/span><\/p>","type":"choose","extra_type":"classic","user_id":"131","test":"0","date":"2023-12-14 01:19:05","option_type":"math","len":0},{"id":"4961","post_id":"7056","mon_id":"1158924","chapter_id":"1159147","question":"<p>Cho c\u1ea5p s\u1ed1 c\u1ed9ng <span class=\"math-tex\">$(u_n)$<\/span> có <span class=\"math-tex\">$u_1=5$<\/span> và t\u1ed5ng c\u1ee7a 50 s\u1ed1 h\u1ea1ng \u0111\u1ea7u b\u1eb1ng 5150. Tìm công th\u1ee9c s\u1ed1 h\u1ea1ng t\u1ed5ng quát <span class=\"math-tex\">$u_n$<\/span>.<\/p>","options":["<strong>A.<\/strong> <span class=\"math-tex\">$u_n=1+4n$<\/span>","<strong>B.<\/strong> <span class=\"math-tex\">$u_n=5n$<\/span>","<strong>C.<\/strong> <span class=\"math-tex\">$u_n=3+2n$<\/span>","<strong>D.<\/strong> <span class=\"math-tex\">$u_n=2+3n$<\/span>"],"correct":"1","level":"3","hint":"","answer":"<p>T\u1ed5ng c\u1ee7a 50 s\u1ed1 h\u1ea1ng \u0111\u1ea7u là 5150 nên<\/p><p><span class=\"math-tex\">$S_{50}=50.u_1+\\dfrac{50.49}{2}.d$<\/span><\/p><p>⇔ <span class=\"math-tex\">$5150=50.5+1225.d$<\/span><\/p><p>⇔ <span class=\"math-tex\">$d=4$<\/span><\/p><p>Khi \u0111ó s\u1ed1 h\u1ea1ng t\u1ed5ng quát <span class=\"math-tex\">$u_n=u_1+(n-1).d=5+(n-1).4=1+4n$<\/span> .<\/p><p>Ch\u1ecdn <span style=\"color:#16a085;\"><strong>A.<\/strong> <span class=\"math-tex\">$u_n=1+4n$<\/span> .<\/span><\/p>","type":"choose","extra_type":"classic","user_id":"131","test":"0","date":"2023-12-14 01:23:38","option_type":"math","len":0}]}
