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ICSE Class X Math 2013 Paper

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Question : 5 of 46
Marks: +1, -0
If (x−2)(x-2) is a factor of the expression 2x3+2 x^{3}+ ax2+bx−14a x^{2}+b x-14 and when the expression is divided by (x−3)(x-3) , it leaves a remainder 52 , find the values of aa and bb .
Solution:  
Let (x−2)(x-2) is a factor of the given expression.
x−2  =0x-2\;=0
x  =2x\;=2
Given: 2x3+ax2+bx−14=02 x^{3}+a x^{2}+b x-14=0
2(2)3+a(2)2+b(2)−14  =02(2)^{3}+a(2)^{2}+b(2)-14\;=0
16+4a+2b−14  =016+4 a+2 b-14\;=0
4a+2b+2  =04 a+2 b+2\;=0
4a+2b  =−24 a+2 b\;=-2
2a+b  =−1    …  (i)  2 a+b\;=-1 \;\; \ldots \;\text{(i)}\;
and when given expression is divided by (x−3)(x-3) .
x−3  =0x-3\;=0
x    =3x \;\; =3
∴    2x3+ax2+bx−14  =52\therefore\;\; 2 x^{3}+a x^{2}+b x-14\;=52
2(3)3+a(3)2+b(3)−66  =02(3)^{3}+a(3)^{2}+b(3)-66\;=0
54+9a+3b−66  =054+9 a+3 b-66\;=0
9a+3b  =129 a+3 b\;=12
3a+b=43 a+b=4 ............(ii)
Solving equation (i) and (ii),
  2a+b=−1\;2 a+b=-1
  3a+b=4\;3 a+b=4
(−)(−)    (−)(-)(-) \;\; (-)
−−−−−−−−--------
−a=−5    ⇒a=5-a=-5 \;\; \Rightarrow a=5
from (ii),
3×5+b  =43 \times 5+b\;=4
b  =4−15=−11b\;=4-15=-11
∴    a=5  and  b=−11\therefore\;\; a=5 \;\text{and}\; b=-11
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