Toán Lớp 7: bài 1 : thực hiện phép tính ( tính nhanh nhất có thể)
a) ( $\frac{-1}{4}$ ) . ( 6$\frac{2}{11}$ ) + 3$\frac{9}{11}$ . ( $\frac{-1}{4}$ )
b) 15$\frac{2}{3}$ . ( $\frac{-1}{5}$ ) + 9$\frac{2}{3}$ . ( $\frac{-1}{5}$ )
c) 15$\frac{3}{4}$ . $\frac{-1}{3}$ + 14$\frac{1}{4}$ . ( $\frac{-1}{3}$ )
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Comments ( 2 )
a)
((-1)/4).(6 2/11)+3 9/11 . ((-1)/4)
=(-1)/4 .(6 2/11+3 9/11)
=(-1)/4 . (6+2/11+3+9/11)
=(-1)/4 . [(6+3)+(2/11+9/11)]
=(-1)/4 . [9+11/11]
=(-1)/4 . (9+1)
=(-1)/4 . 10
=-10/4
=-5/2
b)
15 2/3 . ((-1)/5)+9 2/3 . ((-1)/5)
=(-1)/5 . (15 2/3+9 2/3)
=(-1)/5 . (15+2/3+9+2/3)
=(-1)/5 . [(15+9)+(2/3+2/3)]
=(-1)/5 . (24+4/3)
=(-1)/5 . (72/3+4/3)
=(-1)/5 . 76/3
=-76/15
c)
15 3/4 . ((-1)/3)+14 1/4 . ((-1)/3)
=(-1)/3 . (15 3/4+14 1/4)
=-1/3 . (15+3/4+14+1/4)
=-1/3 . [(15+14)+(3/4+1/4)]
=-1/3 . (29+4/4)
=-1/3 . (29+1)
=-1/3 . 30
=-10