Tìm x để: a) <img alt="De thi vao lop 6 chon mon toan truong THCS Con Thoi nam 2014" width="219" height="35" src="http://images.tuyensinh247.com/picture/article/2014/0805/de-thi-vao...

a) \(x-\frac{1}{2}=\frac{1}{1.2}+\frac{1}{2.3}+...+\frac{1}{19.20}\) \(x-\frac{1}{2}=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+...+\frac{1}{19}-\frac{1}{20}\) \(x-\frac{1}{2}=1-\frac{1}{20}=\frac{19}{20}\) \(x=\frac{19}{20}+\frac{1}{2}=\frac{29}{20}\) Câu trả lời: a) \(x-\frac{1}{2}=\frac{1}{1.2}+\frac{1}{2.3}+...+\frac{1}{19.20}\) \(x-\frac{1}{2}=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+...+\frac{1}{19}-\frac{1}{20}\) \(x-\frac{1}{2}=1-\frac{1}{20}=\frac{19}{20}\) \(x=\frac{19}{20}+\frac{1}{2}=\frac{29}{20}\)

b) (x + 1) + (x + 2) + (x + 3) + ... + (x + 100) = 5750 (x + x + x + ... + x) + (1 + 2 + 3 + ... + 100) = 5750 (100 số x) (100x) + 5050 = 5750 100x = 700 x = 7. Câu trả lời: b) (x + 1) + (x + 2) + (x + 3) + ... + (x + 100) = 5750 (x + x + x + ... + x) + (1 + 2 + 3 + ... + 100) = 5750 (100 số x) (100x) + 5050 = 5750 100x = 700 x = 7.