\(A=\dfrac{1}{3}+\dfrac{1}{6}+\dfrac{1}{10}+...+\dfrac{1}{45}\)
\(=\dfrac{2}{6}+\dfrac{2}{12}+\dfrac{2}{20}+...+\dfrac{2}{90}\)
\(=2\left(\dfrac{1}{6}+\dfrac{1}{12}+\dfrac{1}{20}+...+\dfrac{1}{90}\right)\)
\(=2\left(\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+...+\dfrac{1}{9}-\dfrac{1}{10}\right)\)
\(=2\left(\dfrac{1}{2}-\dfrac{1}{10}\right)=2\cdot\dfrac{2}{5}=\dfrac{4}{5}\)
tính tổng (1-1/3)(1-1/6)(1-1/10)(1-1/15). ... .(1-1/780)
Tính nhanh tổng sau:
A=1/3+1/6+1/10+1/15+1/21+1/28
B=1/5.7+1/7.9+1/9.11+....+1/23.25
\(\text{Tính tổng }A=\frac{1}{2}+\frac{1}{3}+\frac{1}{6}+\frac{1}{10}+\frac{1}{15}+...+\frac{1}{36}+\frac{1}{45}\)
tính tổng
A = 1*99 +2*98 +3*97 +....+ 49*51 + 50*50
B = 1 + 3 + 6 + 10 + 15 +............+ 4950
Tính các tổng một cách hợp lí:
a,42/46+250/286+-2121/2323+-125125/143143
b,4/20+16/42+6/15+-3/5+2/21+-10/21+3/20
c,1/3+1/6+1/10+....+1/210
1) tính tổng
a) S=-3+6-9+12-15+....................................+99
b) A= 5-10 +15+20 + ......................................................+555 -560
1/6+1/10+1/15+1/21+1/28+/36+1/45 (Tính tổng)
1/6+1/10+1/15+1/21+1/28+1/36+1/45(tính tổng)
Tính tổng :S=1+3+6+10+15+......+19600+19998
tính tổng sau = cách hợp lý 1/6 +1/10 +1/15 +.... +1/55
