For natural numbers m, n if (1−y)m(1+y)n=1+a1y+a2y2+...anda1=a2=10,then(m,n) is
A
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B
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C
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D
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Solution
The correct option is B (1−y)m(1+y)n=1+a1y+...+a2y2+... ⇒(1−mC1y+mC2y2−⋯)(1+nC1y+nC−2y2+⋯)=1+a1y+a2y2+⋯ ⇒a1=nC1−mC1,a2=mCn1C1+nC2⇒n−m=10,m2+n2−m−n−2mn=20 ⇒n−m=10,(m−n)2−(m+n)=20 ⇒n−m=10,n+m=10,n+m=80⇒m=35,n=45.