: Deriving the Quadratic Formula Opening Exercise 1. Solve for xx. xx 2 + 2xx = 8 7xx 2 12xx + 4 = 0 Discussion 2. Which of these problems makes more sense to solve by completing the square? Which makes more sense to solve by factoring? How could you tell early in the problem solving process which strategy to use? : Deriving the Quadratic Formula Unit 12: Completing the Square & The Quadratic Formula S.167 This file derived from ALG I--TE-1.3.0-09.2015
3. How would you solve this equation for xx: aaxx + bb = 0, where aa and bb could be replaced with any numbers? 4. Can we say that bb is a formula for solving any equation in the form aaxx + bb = 0? Explain your thinking. aa 5. What would happen if we tried to come up with a way to use just the values of aa, bb, and cc to solve a quadratic equation? Can we solve the general quadratic equation aaxx 2 + bbxx + cc = 0? Is this even possible? Which method would make more sense to use, factoring or completing the square? Deriving the Quadratic Formula You will need: Quadratic Formula Puzzle, scissors, glue stick or tape 6. Work with your partner to put together the Quadratic Formula Puzzle. You will each need your own copy to work with and eventually glue into these notes. There is space on the next page for all of your steps. Be sure they are correct before you glue them in. : Deriving the Quadratic Formula Unit 12: Completing the Square & The Quadratic Formula S.168 This file derived from ALG I--TE-1.3.0-09.2015
aaxx 22 + bbxx + cc = 00 The steps: : Deriving the Quadratic Formula Unit 12: Completing the Square & The Quadratic Formula S.169 This file derived from ALG I--TE-1.3.0-09.2015
Let s test the Quadratic Formula with the problems from the Opening Exercise. 7. Solve for xx, using the Quadratic Formula. Be sure to put each equation in standard form. xx = bb± bb2 4aaaa, for aaxx 2 + bbxx + cc = 0 2aa xx 2 + 2xx = 8 7xx 2 12xx + 4 = 0 a = ; b = ; c = a = ; b = ; c = 8. Did your answers in Exercise 7 agree with your answers in Exercise 1? 9. Let s look a little more closely at the Quadratic Formula. Notice that the whole expression can be split into two separate expressions as follows: bb ± bb 22 44aacc 22aa = bb 22aa ± bb22 44aacc. 22aa This part is the axis of! This part describes the location of the -! : Deriving the Quadratic Formula Unit 12: Completing the Square & The Quadratic Formula S.170 This file derived from ALG I--TE-1.3.0-09.2015
Practice Exercises Use the quadratic formula to solve each equation. 10. xx 2 2xx = 12 aa = 1, bb = 2, cc = 12 (Watch the negatives.) 11. 1 2 rr2 6rr = 2 aa = 1, bb = 6, cc = 2 (Did you remember the negative?) 2 12. 2pp 2 + 8pp = 7 aa = 2, bb = 8, cc = 7 13. 2yy 2 + 3yy 5 = 4 aa = 2, bb = 3, cc = 9 : Deriving the Quadratic Formula Unit 12: Completing the Square & The Quadratic Formula S.171 This file derived from ALG I--TE-1.3.0-09.2015
Solve these quadratic equations, using a different method for each: solve by factoring, solve by completing the square, and solve using the quadratic formula. Before starting, indicate which method you will use for each. 14. Method 15. Method 16. Method 22xx 22 + 55xx 33 = 00 xx 22 + 33xx 55 = 00 11 22 xx22 xx 44 = 00 Lesson Summary The quadratic formula, xx = bb ± bb2 4aaaa, is derived by completing the square on the general 2aa form of a quadratic equation: aaxx 2 + bbxx + cc = 0, where aa 0. The formula can be used to solve any quadratic equation, and is especially useful for those that are not easily solved using any other method (i.e., by factoring or completing the square). : Deriving the Quadratic Formula Unit 12: Completing the Square & The Quadratic Formula S.172 This file derived from ALG I--TE-1.3.0-09.2015
Homework Problem Set Use the quadratic formula to solve each equation. Be sure to write each equation in standard form. 1. Solve for zz: zz 2 3zz 8 = 0. 2. Solve for qq: 2qq 2 8 = 3qq 3. Solve for mm: 1 3 mm2 + 2mm + 8 = 5. 4. Determine the error in Sergio s work below. 0 = 3x 2 4x - 5 2-4± (-4) -4(3)(-5) x = 2(3) -4± 16+60 x = 6-4± 76-4±2 19-2± 19 x = = = 6 6 3-2+ 19-2- 19 x= or x= 3 3 : Deriving the Quadratic Formula Unit 12: Completing the Square & The Quadratic Formula S.173 This file derived from ALG I--TE-1.3.0-09.2015
Solve these quadratic equations, using a different method for each: solve by factoring, solve by completing the square, and solve using the quadratic formula. Before starting, indicate which method you will use for each. 5. Method 6. Method 7. Method 3xx 2 + 13xx 10 = 0 xx 2 12xx + 28 = 0 2xx 2 9xx 9 = 0 8. Geometry Connection Determine the base and height of the right triangle below. Its area is 8 cm 2. [source: http://www.projectmaths.ie/documents/t&l/introductiontoquadraticequations.pdf] x + 2 2x : Deriving the Quadratic Formula Unit 12: Completing the Square & The Quadratic Formula S.174 This file derived from ALG I--TE-1.3.0-09.2015