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If (3+22)(3+2\sqrt{2}) is a root of the equation ax2+bx+c=0ax^2 + bx + c = 0, and (4+23)(4+2\sqrt{3}) is a root of the equation ay2+my+n=0ay^2 + my + n = 0, where a,b,c,ma, b, c, m and nn are integers,
then the value of (bm+c−2bn)(\frac{b}{m} + \frac{c-2b}{n}) is

Solution

✅ Correct Option: 4

When a quadratic equation with integer coefficients has an irrational root containing a square root, the conjugate of that root must also be a root.

If we have a+bka + b\sqrt{k} as a root where a,ba, b are rational and kk is not a perfect square, then a−bka - b\sqrt{k} must also be a root. This ensures that when we expand the quadratic, all coefficients remain integers since the irrational parts cancel out.


For the first equation: ax2+bx+c=0ax^2 + bx + c = 0

Given root: (3+22)(3+2\sqrt{2})

Missing root: (3−22)(3-2\sqrt{2}) (conjugate)

For the second equation: ay2+my+n=0ay^2 + my + n = 0

Given root: (4+23)(4+2\sqrt{3})

Missing root: (4−23)(4-2\sqrt{3}) (conjugate)


First equation with roots (3+22)(3+2\sqrt{2}) and (3−22)(3-2\sqrt{2}):

Sum of roots: (3+22)+(3−22)=6(3+2\sqrt{2}) + (3-2\sqrt{2}) = 6

Product of roots: (3+22)(3−22)(3+2\sqrt{2})(3-2\sqrt{2})

Using the identity (a+b)(a−b)=a2−b2(a+b)(a-b) = a^2 - b^2:

=32−(22)2=9−8=1= 3^2 - (2\sqrt{2})^2 = 9 - 8 = 1

Therefore: x2−6x+1=0x^2 - 6x + 1 = 0


Second equation with roots (4+23)(4+2\sqrt{3}) and (4−23)(4-2\sqrt{3}):

Sum of roots: (4+23)+(4−23)=8(4+2\sqrt{3}) + (4-2\sqrt{3}) = 8

Product of roots: (4+23)(4−23)(4+2\sqrt{3})(4-2\sqrt{3})

=42−(23)2=16−12=4= 4^2 - (2\sqrt{3})^2 = 16 - 12 = 4

Therefore: x2−8x+4=0x^2 - 8x + 4 = 0


From x2−6x+1=0x^2 - 6x + 1 = 0:

a=1a = 1, b=−6b = -6, c=1c = 1

From x2−8x+4=0x^2 - 8x + 4 = 0:

a=1a = 1, m=−8m = -8, n=4n = 4


Substituting into (bm+c−2bn)\left(\dfrac{b}{m} + \dfrac{c-2b}{n}\right):

bm=−6−8=68=34\dfrac{b}{m} = \dfrac{-6}{-8} = \dfrac{6}{8} = \dfrac{3}{4}

c−2bn=1−2(−6)4=1+124=134\dfrac{c-2b}{n} = \dfrac{1-2(-6)}{4} = \dfrac{1+12}{4} = \dfrac{13}{4}

Therefore: (bm+c−2bn)=34+134=164=4\left(\dfrac{b}{m} + \dfrac{c-2b}{n}\right) = \dfrac{3}{4} + \dfrac{13}{4} = \dfrac{16}{4} = 4


Answer: D) 4

When we work with quadratic equations having integer coefficients and irrational roots, we should always remember that irrational roots come in conjugate pairs. This property allows us to quickly form the complete quadratic equation and find all coefficients.

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