Question

tính nhanh:

9/15x18+9/18x21+9/21x24+...+9/87x90

Answer 1

Gọi tổng bằng S

1/9 S=1/15*18+..................+1/87*90

1/9 S=1/15-1/18+1/18-......................-1/90

1/9 S=1/15-1/90

1/9 S=1/18

S=9/18=1/2