Question

[Công Chúa Họ NGuyễn]

Tính ;

a; \(\frac{3}{2}\)+\(\frac{3}{6}\)+\(\frac{3}{12}\)+...+\(\frac{3}{90}\)

b; \(\frac{1}{12}\)+\(\frac{1}{24}\)+\(\frac{1}{40}\)+...\(\frac{1}{84}\)

help me!

Answer 1

[Công chúa 123]

a, 3/2 + 3/6 + 3/12 + . . . + 3/90

= 3/1*2 + 3/2*3 + 3/3*4 + . . . + 3/9*10

= 1/1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + . . . + 1/9 - 1/10

= 1/1 - 1/10 = 9/10

Vậy a = 9/10

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