ICSE Class X Math 2016 Solved Paper

Question : 20 of 57

Marks: +1, -0

SECTION - II

Solve the quadratic equation

x^2-3(x+3)=

0 ; Give your answer correct to two significant figures.

Solution:

Given :

x^2-3(x+3)=0

x^2-3 x-9=0

Comparing

x^2-3 x-9=0

with

a x^2+b x+c

=0

, we get
We know,

a=1, b=-3, c=-9

x\;=\;{-b ± {√{b^2-4 a c}}}/{2 a}

\;=\;{-(-3) ± {√{(-3)^2-4 × 1 ×(-9)}}}/{2 × 1}

\;=\;{3 ± {√{9+36}}}/{2}=\;{3 ± {√{45}}}/{2}

\;=\;{3 ± 3 {√{5}}}/{2}=\;{3 ± 3 × 2.236}/{2}

x\;=\;{3 ± 6.708}/{2}

x\;=\;{3+6.708}/{2} \;\text " and "\; \;{3-6.708}/{2}

x\;=4.85 \;\text ", and "\;-1.85

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