Medium3 marksStructured
NumberRecurring DecimalsFractionsProofNumber

AQA GCSE · Question 21 · Number

Prove algebraically that 1.018 (with 18 recurring) = 56/55

How to approach this question

1. Set the recurring decimal equal to a variable, say `x`. So, `x = 1.01818...` 2. Multiply `x` by powers of 10 to create two new equations where the decimal parts after the point are identical. For example, find `10x` and `1000x`. 3. Subtract one equation from the other. This should result in an equation with no recurring decimals. 4. Solve the resulting equation for `x`. 5. Simplify the fraction to its lowest terms to show it equals 56/55.

Full Answer

Let x = 1.0181818... Step 1: Multiply x by a power of 10 to move the decimal point past the start of the recurring part. 10x = 10.181818... Step 2: Multiply x by another power of 10 to move the decimal point past the end of the first block of the recurring part. The recurring part has 2 digits (18), so we multiply 10x by 100 (or x by 1000). 1000x = 1018.181818... Step 3: Subtract the smaller equation from the larger one to eliminate the recurring decimal part. 1000x = 1018.181818... - 10x = 10.181818... -------------------- 990x = 1008 Step 4: Solve for x. x = 1008 / 990 Step 5: Simplify the fraction. Divide by 2: 504 / 495 The sum of digits of 504 is 9, and for 495 is 18. Both are divisible by 9. 504 / 9 = 56 495 / 9 = 55 So, x = 56/55. The proof is complete.
To prove that a recurring decimal is equal to a certain fraction, we must convert the decimal into a fraction algebraically. 1. Let `x` be the recurring decimal: `x = 1.0181818...` 2. We want to create two equations where the part after the decimal point is the same, so we can subtract them and eliminate the recurring part. The recurring part is `18`. 3. Multiply `x` by 10 to move the decimal point just before the recurring part begins: `10x = 10.181818...` (Equation 1) 4. Multiply `x` by 1000 to move the decimal point after the first full cycle of the recurring part: `1000x = 1018.181818...` (Equation 2) 5. Now, subtract Equation 1 from Equation 2: `1000x - 10x = 1018.181818... - 10.181818...` `990x = 1008` 6. Solve for `x` by dividing by 990: `x = 1008 / 990` 7. Simplify the fraction. We can divide both the numerator and denominator by their common factors. - Divide by 2: `x = 504 / 495` - The sum of the digits of 504 is 9, and the sum of the digits of 495 is 18. Both are divisible by 9. - 504 ÷ 9 = 56 - 495 ÷ 9 = 55 - So, `x = 56/55`. We have shown that 1.018 (recurring) is equal to 56/55.

Common mistakes

✗ Choosing the wrong powers of 10 to multiply by (e.g., 10x and 100x). ✗ Making a mistake when subtracting the numbers or the algebraic terms. ✗ Failing to simplify the resulting fraction correctly.

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