Toán Lớp 6: cho x-y=7 tính A=3(x-5y+1)-3(y-5x-2)+1986-10(x-y+3)
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Toán Lớp 6: cho x-y=7 tính A=3(x-5y+1)-3(y-5x-2)+1986-10(x-y+3)
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A = 3(x – 5y + 1) – 3(y – 5x – 2) + 1986 – 10(x – y + 3)
=> A = 3[(x – 5y + 1) – (y – 5x – 2)] + 1986 – 10(x – y + 3)
=> A = 3(x – 5y + 1 – y + 5x + 2) + 1996 – 10(x – y + 3)
=> A = 3(6x – 6y + 3) + 1986 – 10(x – y + 3)
=> A = 3[6(x – y) + 3] + 1986 – 10(x – y + 3)
Thay x – y = 7 ta có :
A = 3(6 . 7 + 3) + 1986 – 10(7 + 3)
=> A = 3(42 + 3) + 1986 – 10.10
=> A = 3.45 + 1986 – 100
=> A= 135 + 1986- 100
=> A = 2121 – 100
=> A = 2021
Vậy A = 2021
#dtkc
A = 3 . ( x – 5y + 1 ) – 3 . ( y – 5x – 2 ) + 1986 – 10 . ( x – y + 3 )
A = 3 . [ ( x – 5y + 1 ) – ( y – 5x – 2 ) ] + 1986 – 10 . ( x – y + 3 )
A = 3 . [ x – 5y + 1 – y + 5x + 2 ] + 1986 – 10 . ( x – y + 3 )
A = 3 . [ 6x – 6y + 3 ] + 1986 – 10 . ( x – y + 3 )
A = 3 . [ 6 . ( x – y ) + 3 ] + 1986 – 10 . ( x – y + 3 )
Thay x – y = 7 , ta được :
A = 3 . [ 6 . 7 + 3 ] + 1986 – 10 . ( 7 + 3 )
A = 3 . [ 42 + 3 ] + 1986 – 10 . 10
A = 3 . 45 + 1986 – 100
A = 135 + 1986 – 100
A = 2121 – 100
A = 2021
Vậy , A = 2021 .