Explanation
Coefficient of $$x^r $$ and $$x^{r+1} $$ are equal.
Coefficient of $$T_{r+1} =$$ Coefficient of $$T_{r+2} $$
$$\Rightarrow\displaystyle { ^{ 55 }C }_{ r }{ (2) }^{ 55-r }{ \left(\dfrac { 1 }{ 3 } \right) }^{ r }={ ^{ 55 }C }_{ r+1 }{ (2) }^{ 54-r }{ \left(\dfrac { 1 }{ 3 } \right) }^{ r+1 }$$
. $$\dfrac { 55! }{ (55-r)!r! } (2)=\dfrac { 55! }{ (54-r)!(r+1)! } \dfrac { 1 }{ 3 } $$
$$\Rightarrow 6(r+1) = 55-r$$
$$\Rightarrow r =7$$
So, the terms will be $$8^{th}$$ and $$9^{th}$$
Consider given the expression,
$${{\left( x+y \right)}^{50}}+{{\left( x-y \right)}^{50}}$$
We know that,
$${{\left( x+y \right)}^{n}}={}^{n}{{C}_{0}}{{x}^{n}}{{+}^{n}}{{C}_{1}}{{x}^{n-1}}y{{+}^{n}}{{C}_{2}}{{x}^{n-2}}{{y}^{2}}+{{......}^{n}}{{C}_{r}}{{x}^{n-r}}{{y}^{r}}{{.....}^{n}}{{C}_{n}}{{y}^{n}}$$
Now,
$${{\left( x+y \right)}^{50}}+{{\left( x-y \right)}^{50}}=2{{[}^{50}}{{C}_{0}}{{x}^{50}}{{+}^{50}}{{C}_{2}}{{x}^{50-2}}y{{+}^{50}}{{C}_{4}}{{x}^{50-4}}{{y}^{4}}+{{......}^{50}}{{C}_{r}}{{x}^{50-r}}{{y}^{r}}{{.....}^{50}}{{C}_{50}}{{y}^{50}}$$
Hence, there are $$26$$ terms
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