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Finding the Unknown - A Pinch of History

Grade 7CBSE

Review the key concepts, formulae, and examples before starting your quiz.

๐Ÿ”‘Concepts

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The word 'Algebra' is derived from the title of the book 'Al-Kitab al-mukhtasar fi hisab al-jabr waโ€™l-muqabala' written by the Persian mathematician Al-Khwarizmi. 'Al-Jabr' means the restoration of broken parts.

โ€ข

Indian mathematicians like Aryabhata and Brahmagupta used 'Beejganit' to refer to algebra. They developed methods to solve linear equations of the form ax+b=cax + b = c.

โ€ข

An algebraic equation is a condition on a variable. It says that two expressions are equal. For example, in 4x+5=254x + 5 = 25, the Left Hand Side (LHS) is 4x+54x + 5 and the Right Hand Side (RHS) is 2525.

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Balancing Method: An equation remains unchanged if the same number is added to or subtracted from both sides, or if both sides are multiplied or divided by the same non-zero number.

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Transposition Method: Moving a term from one side of the equation to the other is called transposition. During transposition, the sign of the term changes: ++ becomes โˆ’-, โˆ’- becomes ++, ร—\times becomes รท\div, and รท\div becomes ร—\times.

๐Ÿ“Formulae

ax+b=cax + b = c

x=cโˆ’bax = \frac{c - b}{a}

x+a=bโ€…โ€ŠโŸนโ€…โ€Šx=bโˆ’ax + a = b \implies x = b - a

xโˆ’a=bโ€…โ€ŠโŸนโ€…โ€Šx=b+ax - a = b \implies x = b + a

xa=bโ€…โ€ŠโŸนโ€…โ€Šx=bร—a\frac{x}{a} = b \implies x = b \times a

๐Ÿ’กExamples

Problem 1:

Solve for xx: 3x+12=303x + 12 = 30.

Solution:

3x=30โˆ’123x = 30 - 12 3x=183x = 18 x=183x = \frac{18}{3} x=6x = 6

Explanation:

We first transpose +12+12 to the RHS, where it becomes โˆ’12-12. Then, we transpose the coefficient 33 (which is multiplying xx) to the RHS, where it divides 1818.

Problem 2:

A number yy is such that when 77 is subtracted from it, the result is 1515. Find yy.

Solution:

yโˆ’7=15y - 7 = 15 y=15+7y = 15 + 7 y=22y = 22

Explanation:

The problem is translated into the equation yโˆ’7=15y - 7 = 15. By transposing โˆ’7-7 to the RHS, we add it to 1515 to find the value of yy.

Problem 3:

Find the value of xx in the equation x+34567892=80000000x + 34567892 = 80000000.

Solution:

x=80000000โˆ’34567892x = 80000000 - 34567892 80000000โˆ’3456789245432108\begin{array}{r} 80000000 \\ -34567892 \\ \hline 45432108 \end{array} x=45432108x = 45432108

Explanation:

We transpose the large constant to the RHS and perform vertical subtraction to find the unknown variable xx.