Toán Lớp 7: 1.cho x/2=y/3=z/5 tính A= 3x-y+5x/x+y+3x 2. x/4=y/7=z/5 Tính B= 2x+y-z/x+6y-5z

Question

Toán Lớp 7:  1.cho x/2=y/3=z/5 tính A= 3x-y+5x/x+y+3x 2. x/4=y/7=z/5 Tính B= 2x+y-z/x+6y-5z

lyly98 · Answer

Giải đáp:   $  $   1,   Đặt x/2 = y/3 = z/5 = k   &harr;   ( left {  begin{array}{l} dfrac{x}{2}=k    dfrac{y}{3}=k    dfrac{z}{5}=k end{array}  right. )     &harr;  ( left {  begin{array}{l}x=2k  y=3k  z=5k end{array}  right. )     C&oacute; : A = (3x-y + 5x)/(x+y+3x)     Thay  ( left {  begin{array}{l}x=2k  y=3k  z=5k end{array}  right. ) v&agrave;o A ta được :     &harr; A = (3 (2k) - 3k +5 (2k) )/(2k + 3k + 3 (2k) )     &harr; A = (6k - 3k + 10k)/(2k + 3k + 6k)     &harr; A = (k(6-3 +10) )/(k (2+3+6) )     &harr; A = (6-3+10)/(2+3+6)     &harr; A = (3+10)/(5+6)     &harr;A = 13/11     Vậy A = 13/11 khi x/2=y/3=z/5     $  $     $  $     2,     Đặt x/4 = y/7 = z/5 = k     &harr;    ( left {  begin{array}{l} dfrac{x}{4}=k    dfrac{y}{7}=k    dfrac{z}{5}=k end{array}  right. )     &harr;  ( left {  begin{array}{l}x=4k  y=7k  z=5k end{array}  right. )     C&oacute; : B = (2x + y - z)/(x+6y - 5z)     Thay  ( left {  begin{array}{l}x=4k  y=7k  z=5k end{array}  right. ) v&agrave;o B ta được  :     &harr; B = (2 (4k) + 7k - 5k)/(4k + 6 (7k) - 5 (5k) )     &harr; B = (8k + 7k-5k)/(4k + 42k - 25k)     &harr; B = (k (8+7-5) )/(k (4+42-25) )     &harr; B=(8+7-5)/(4+42-25)     &harr; B = (15 - 5)/(46 - 25)     &harr; B = 10/21     Vậy B=10/21 khi x/4=y/7=z/5

ankim · Answer

B&agrave;i 1:   Đặt x/2 = y/3 = z/5 =k   => x= 2k ; y= 3k ; z= 5k (1)   Thay (1) v&agrave;o A ta được:   A= (3. 2k - 3k + 5 . 2k)/( 2k + 3k + 3. 2k)   A= (6k - 3k + 10k)/(5k + 6k)   A= (13k)/(11k)   A= 13/11   Vậy A= 13/11   B&agrave;i 2:   Đặt x/4 = y/7 = z/5 =k   => x = 4k ; y= 7k ; z= 5k (1)   Thay (1) v&agrave;o B ta được   B= ( 2. 4k+ 7k - 5k)/(4k + 6. 7k - 5.5k)   B= (8k + 7k - 5k)/(4k + 42 k - 25k)   B=( 15k -5k)/(46k - 25k)   B=(10k)/(21k)   B=10/21   Vậy B= 10/21