Question

[Lê Viết Hiệp]

chứng minh :91/1*4+91/4*7+61/7*10+.......91/88*91=30

nếu đúng mình tick luôn

Answer 1

[Xyz OLM]

Ta có \(\frac{91}{1.4}+\frac{91}{4.7}+\frac{91}{7.10}+...+\frac{91}{88.91}=\frac{91}{3}.\left(\frac{3}{1.4}+\frac{3}{4.7}+\frac{3}{7.10}+...+\frac{3}{88.91}\right)\)

\(=\frac{91}{3}.\left(1-\frac{1}{4}+\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+...+\frac{1}{88}-\frac{1}{91}\right)\)

\(=\frac{91}{3}.\left(1-\frac{1}{91}\right)=\frac{91}{3}.\frac{90}{91}=30\)

Answer 2

[✦๖ۣۜAugųsť❦❄]

9/1.4+9/4.7+9/7.10+…+9/88.91=30

=91/3. (3/1.4+3/4.7+3/7.10+…+3/88.91)

=91/3. (1-1/4+1/4-1/7+1/7-1/10+…+1/88-1/91)

=91/3*90/91=30