Bài 7.4:Tính hợp lí:
a,A=54+(-37+10-54+67)
b,B=1+2-3-4+5+6-7-8+……-79-80+81
Bài 7.4:Tính hợp lí: a,A=54+(-37+10-54+67) b,B=1+2-3-4+5+6-7-8+……-79-80+81
By Hadley
By Hadley
Bài 7.4:Tính hợp lí:
a,A=54+(-37+10-54+67)
b,B=1+2-3-4+5+6-7-8+……-79-80+81
Đáp án:
`a)`
` A = 54 + (-37 +10 + 67 – 54)~
` = 54 – 37 + 10 + 67 – 54`
` = (54-54) + (67-37) +10`
` = 0 + 30 + 10`
` = 40`
`b)`
` B = 1 +2 – 3 – 4 + 5 + 6 – 7 -8 + ….. -79 – 80 +81`
` = (1 +2 – 3 -4) + (5+6-7-8) + …… + (77+78-79-80) +81`
` = -4 + (-4) + …… + (-4) + 81` ( có `20` số `(-4)` )
` = 20 * (-4) +81`
` = -80 +81`
` = 1`
a,A = 54 + ( – 37 + 10 – 54 + 67 )
A = 54 + ( – 37 ) + 10 – 54 + 67
A = [ 54 – 54 ] + [ ( – 37 ) + 67 ] + 10
A = 0 + 30 + 10
A = 40
b. B = 1+ 2 – 3 – 4 + 5 – 6 + 7 – 8 + …. – 79 – 80 + 81
B = (1 + 2 – 3 – 4) + (5 + 6 – 7 – 8) +…+ (77 + 78 – 79 – 80) + 81
B = ( – 4 ) + ( -4 ) +…+ ( -4 ) – 81
B = ( – 4 ) . 20 + 81
B = ( – 80 )+ 81
B = 1