Câu 2. Tính S=2x-y+z biết: x:y:z=3:4:5 và x+y-z=28

1. **Express variables in terms of a common factor:**<br /> Since $x:y:z=3:4:5$, we can write:<br /> * $x = 3k$<br /> * $y = 4k$<br /> * $z = 5k$ (where k is a constant)<br /><br />2. **Substitute into the given equation:**<br /> Substitute the expressions for x, y, and z into the equation $x + y - z = 28$:<br /> $3k + 4k - 5k = 28$<br /><br />3. **Solve for k:**<br /> Simplify the equation: $2k = 28$<br /> Therefore, $k = 14$<br /><br />4. **Calculate x, y, and z:**<br /> * $x = 3k = 3 * 14 = 42$<br /> * $y = 4k = 4 * 14 = 56$<br /> * $z = 5k = 5 * 14 = 70$<br /><br />5. **Calculate S:**<br /> Substitute the values of x, y, and z into the expression for S:<br /> $S = 2x - y + z = 2(42) - 56 + 70 = 84 - 56 + 70 = 98$<br /><br />Therefore, $S = 98$. <br />