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EXAMPLE 2. Evaluate a variable expression. Evaluate the expression y – x + 6.8 when x = – 2 and y = 7.2. y – x + 6.8 = 7.2 – ( –2 ) + 6.8. Substitute – 2 for x and 7.2 for y. = 7.2 + 2 + 6.8. Add the opposite – 2. = 16. Add. EXAMPLE 3. Evaluate Change. Temperature.
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EXAMPLE 2 Evaluate a variable expression Evaluate the expression y – x + 6.8 when x = – 2 and y = 7.2. y–x+ 6.8 =7.2– (–2)+ 6.8 Substitute –2 for xand 7.2 for y. =7.2 + 2 +6.8 Add the opposite –2. = 16 Add.
EXAMPLE 3 Evaluate Change Temperature One of the most extreme temperaturechanges in United States history occurred in Fairfield, Montana, on December 24, 1924. At noon, the temperature was 63°F.By midnight, the temperature fell to – 21°F. What was the change in temperature? SOLUTION The change Cin temperature is the difference of the temperature mat midnight and the temperature nat noon.
C = – m n m – n C = EXAMPLE 3 Evaluate Change STEP1 Write a verbal model. Then write an equation. STEP2 Find the change in temperature. Write equation. = – 21 – 63 Substitute values. = – 21 + (– 63) Add the opposite of 63. = – 84 Add –21and –63.
ANSWER The change in temperature was – 84°F. EXAMPLE 3 Evaluate Change
7. Car values x – y + 8 y – (x –2) (y – 4)– x 4. 5. 6. ANSWER – $2700 for Example 2 and 3 GUIDED PRACTICE Evaluate the expression when x = – 3 and y = 5.2. =–0.2 =10.2 =4.2 A new car is valued at $15,000. One year later, the car is valued at $12,300. What is the change in value of car?