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SubjectsMathematics
बीजीय व्यंजक
Algebraic Expressions introduces Class 9 Mathematics students to the language used for representing numbers and relationships with variables. As part of the chapter Introduction to Polynomials, students learn to identify terms, coefficients, constants, and variables; distinguish like and unlike terms; and simplify expressions by combining like terms. They also practise substituting values to evaluate expressions and applying addition, subtraction, multiplication, and division carefully, building a foundation for understanding polynomials and solving algebraic problems.
Correct answer: B. 19
Explanation: Substitute -3 for x: \(x^2-2x+4=(-3)^2-2(-3)+4\). This gives \(9+6+4=19\), so 19 is correct. Option 7 may result from incorrectly treating \(-2(-3)\) as -6; the product of two negative numbers is positive. Exam tip: Always use brackets when squaring or substituting a negative value.
Correct answer: A. \(4a+9b\)
Explanation: Combine like terms in the expression: \(6a-2a=4a\) and \(4b+5b=9b\). Therefore, the simplified form is \(4a+9b\). In \(8a+9b\), \(6a\) and \(-2a\) have been added incorrectly. Exam tip: Add or subtract coefficients only for terms with the same variable and the same power.
Correct answer: C. \(p^2+4p\)
Explanation: The square of the number \(p\) is \(p^2\), and four times the number is \(4p\). Since the question asks to add four times the number to its square, the expression is \(p^2+4p\). Option B uses subtraction, while option D multiplies the entire sum by 4. Exam tip: For phrases such as “add to,” use \(+\) and write the square and the multiple separately first.
Correct answer: D. \(6m^2\)
Explanation: The like terms containing \(m^2\) are \(9m^2\) and \(-3m^2\). Adding their coefficients gives \(9-3=6\), so the result is \(6m^2\). The term \(7m\) cannot be combined with them because its power of \(m\) is 1, not 2. Exam tip: combine terms only when both the variable and its exponent are the same.
Correct answer: A. -2
Explanation: Given \(a=2\) and \(b=-4\), \(ab+3a=(2)(-4)+3(2)=-8+6=-2\). Therefore, option A is correct. The value \(-14\) may result from incorrectly treating \(3a\) as \(-6\). Exam tip: after substitution, check the signs carefully while multiplying and adding negative numbers.
Correct answer: B. \(5x+5\)
Explanation: A minus sign before the bracket changes the sign of every term inside it: \(8x-(3x-5)=8x-3x+5\). Combining like terms gives \(8x-3x=5x\), so the simplified expression is \(5x+5\). The option \(5x-5\) results from incorrectly keeping the sign of \(-5\) unchanged. Exam tip: when removing brackets preceded by ‘−’, reverse every sign inside the brackets.
Correct answer: A. \(4x^2-3x+7\)
Explanation: \(4x^2-3x+7\) has three terms: \(4x^2\), \(-3x\), and \(7\), so it is a trinomial. \(5a-2\) has only two terms. Exam tip: count terms separated by + or − signs.
Correct answer: D. \(5y^2+3y-7\)
Explanation: Combine like terms: \(2y^2+3y^2=5y^2\) and \(5y-2y=3y\). The constant term \(-7\) remains unchanged. Therefore, the simplified expression is \(5y^2+3y-7\). In option C, the sign of the constant term is incorrect. Exam tip: add or subtract only terms with the same variable and the same exponent.
Correct answer: A. \(1\)
Explanation: In the expression \(12p^2q\), no exponent is written on \(q\). Therefore, \(q=q^1\), so the power of \(q\) is \(1\). The exponent \(2\) belongs to \(p\), not to \(q\). Exam tip: A variable with no written exponent always has exponent \(1\).
Correct answer: C. \(x+y-6\)
Explanation: The sum of \(x\) and \(y\) is \(x+y\). Taking 6 less than this sum means subtracting 6 from it, giving \(x+y-6\). The expression \(x+y+6\) adds 6, whereas the question asks for 6 less. Exam tip: For “less than” in this form, subtract the stated number from the expression already formed.
Correct answer: A. 10n-2
Explanation: Expanding the brackets gives 4(3n+2)=12n+8 and -2(n+5)=-2n-10. Therefore, 12n+8-2n-10 = 10n-2. Option B results from using the wrong sign for the constant term in the second bracket. Exam tip: multiply the number before a negative bracket by both terms inside it.
Correct answer: C. They are like terms
Explanation: Both (3ab^2) and (-10ab^2) have the variable part ab^2: the power of a is 1 and the power of b is 2. Only their coefficients, 3 and -10, are different, so they are like terms. Options B and D are incorrect because like terms may have different coefficients, but their variables and respective powers must match. Exam tip: Ignore coefficients first, then compare every variable and its exponent to identify like terms.
Correct answer: B. 11
Explanation: Substituting t=5 gives t^2-4t+6=5^2-4(5)+6=25-20+6=11. Therefore, 11 is the correct answer. The value 9 may result from incorrectly omitting the final +6. Exam tip: substitute the value first, then follow the order of powers, multiplication, and addition/subtraction.
Correct answer: A. \(21xy+5x\)
Explanation: Here, \(15xy\) and \(6xy\) are like terms, so their sum is \(21xy\). Similarly, \(-4x\) and \(9x\) are like terms and add to \(5x\). Therefore, the simplified expression is \(21xy+5x\). In option B, the sign while combining the \(x\)-terms is incorrect. Exam tip: Add or subtract only terms that have exactly the same variables with the same powers.
Correct answer: C. -6
Explanation: In the expression
\(-6x^2+11x-3\), the term containing
\(x^2\) is
\(-6x^2\). Therefore, the coefficient of
\(x^2\) is
\(-6\). Option 6 is incorrect because the negative sign is part of the coefficient. Exam tip: Include the sign when identifying the numerical factor multiplying a variable term.
Correct answer: A. \(2r+3s\)
Explanation: Twice \(r\) is \(2r\), and three times \(s\) is \(3s\). Adding them gives \(2r+3s\), so option A is correct. In \(3r+2s\), the coefficients are interchanged, while expanding \(2(r+3s)\) gives \(2r+6s\). Exam tip: Write the stated multiple with each variable first, then use \(+\) for “sum.”
Correct answer: C. 6
Explanation: Substituting the given values, \(u^2-uv=(-2)^2-(-2)(1)=4-(-2)=4+2=6\). Hence, the correct answer is 6. The value 4 represents only \(u^2\); subtracting \(uv=-2\) gives 6. In exams, remember that subtracting a negative quantity changes it to addition.
Correct answer: B. \(6x^3-8x^2\)
Explanation: Use the distributive property: \(2x^2\times 3x=6x^3\) and \(2x^2\times(-4)=-8x^2\). Therefore, the expanded expression is \(6x^3-8x^2\). In option D, the second term is positive, but multiplication by \(-4\) gives a negative term. Exam tip: while multiplying powers of \(x\), add their exponents; for example, \(x^2\times x=x^3\).
Correct answer: A. \(7p^2q-3pq\)
Explanation: \(5p^2q\) and \(2p^2q\) are like terms, so adding their coefficients gives \((5+2)p^2q=7p^2q\). The term \(-3pq\) has variable part \(pq\), which is different from \(p^2q\), so it cannot be combined with \(7p^2q\). Therefore, the simplified expression is \(7p^2q-3pq\). Exam tip: combine only terms with exactly the same variables raised to the same powers.
Correct answer: B. \(-5x+9\)
Explanation: Combining the like variable terms gives \(4x-9x=-5x\), and the constant terms give \(7+2=9\). Therefore, the simplified expression is \(-5x+9\). In \(5x+9\), the sign of \(4x-9x\) has been handled incorrectly. Exam tip: add or subtract only like terms.
Correct answer: C. 7(x-2)
Explanation: Using the distributive property, \(7(x-2)=7\times x-7\times 2=7x-14\), so option C is correct. Option A expands to \(7x+14\), while option B expands to \(14x-14\); neither matches the given expression. Exam tip: multiply the number outside the bracket by every term inside it.
Correct answer: C. x and y
Explanation: In the term \(3x^2y^2\), \(x\) and \(y\) are letters whose values can vary, so they are variables. The number \(3\) is the numerical coefficient, while \(2\) is an exponent; neither is a variable. Exam tip: identify the letters in a term, but do not count their exponents as variables.
Correct answer: A. 5
Explanation: Substituting k=3 gives 2(3+4)-3^2. The bracket equals 7, so 2×7=14, and 3^2=9. Therefore, 14-9=5, so 5 is correct. The value 14 is only the result of the multiplication part; 9 still has to be subtracted. Exam tip: after substitution, evaluate brackets, powers, multiplication, and then subtraction in order.
Correct answer: B. \(6a^2+2a\)
Explanation: Combine like terms: \(11a^2-5a^2=6a^2\) and \(-4a+6a=2a\). Therefore, the simplified expression is \(6a^2+2a\). In option A, the coefficients of the squared terms have been added incorrectly. Exam tip: Add or subtract only terms that have the same variable with the same exponent.
Correct answer: C. \(2(x^2+5)\)
Explanation: First, square \(x\) and add 5 to get \(x^2+5\). Twice this entire sum is \(2(x^2+5)\), so option C is correct. In option A, only \(x^2\) is doubled, not 5. Exam tip: When you read “twice the sum,” put the complete sum inside brackets.
