数学题8到简算 要写清楚点 小学滴

9. 7/9×2/13+11/9×7/13
=(7*2)/(9*13)+(11*7)(9*13)
=(7*13)/(9*13)
=7/9
10. 3/8×7+0.375
=(3/8)*7+3/8
=(3/8)(7+1)
=3

11. 1/1×3+ 1/3×5+ 1/5×7+ 1/7×9
=(1/2)(1-1/3+1/3-1/5+1/5-1/7+1/7-1/9)
=(1/2)(1-1/9)
=4/9

12. 1/3×1/8+1/8×1/13+1/13×1/18
=(1/5)(1/3-1/8+1/8-1/13+1/13-1/18)
=1/5(1/3-1/18)
=1/18

13. 1/4-1/8-1/16-1/32-1/64
=(16-8-4-2-1)/64
=1/64
14. 1/4+1/8+1/16+1/32+1/64
=(16+8+4+2+1)/64
=31/64

15. 1/6+1/12+1/20+1/30
=1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6
=1/2-1/6
=1/3

16. 7/6+13/12+21/20+31/30
=1+1/6+1+1/12+1+1/20+1+1/30
=4+(1/6+1/12+1/20+1/30)
=4+(1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6)
=4+(1/2-1/6)
=4+1/3
=13/4

是乘以的意思啦
9. 7/9×2/13+11/9×7/13
=(7×2)/(9×13)+(11×7)(9×13)
=(7×13)/(9×13)
=7/9
10. 3/8×7+0.375
=(3/8×7+3/8
=(3/8)(7+1)
=3