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Mathematics

If a : b = c : d, then ma+ncmb+nd\dfrac{ma + nc}{mb + nd} is equal to :

  1. m : n

  2. dm : cn

  3. an : mb

  4. a : b

Ratio Proportion

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Answer

Given,

⇒ a : b = c : d

ab=cd\Rightarrow \dfrac{a}{b} = \dfrac{c}{d}

Let, ab=cd\dfrac{a}{b} = \dfrac{c}{d} = k where k is the constant ratio.

Therefore, a = bk and c = dk

Substituting values of a and c in ma+ncmb+nd\dfrac{ma + nc}{mb + nd}, we get :

m(bk)+n(dk)mb+ndk(mb+nd)mb+ndkaba:b.\Rightarrow \dfrac{m(bk) + n(dk)}{mb + nd} \\[1em] \Rightarrow \dfrac{k(mb + nd)}{mb + nd} \\[1em] \Rightarrow k \\[1em] \Rightarrow \dfrac{a}{b} \\[1em] \Rightarrow a : b.

Hence, option 4 is the correct option.

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