Use the Binomial Expansion Formula: (a + b)n = nCr-1 * a [n -(r - 1)] * b r - 1  to determine the 4th term

With a = x;  b = y  ;  r, or term number = 4; and n = 7,

(a + b)n = nCr-1 * a [n -(r - 1)] * b r - 1  becomes:

(x + y)7 = 7C4-1 * x [7 -(4 - 1)] * y 4 - 1 

(x + y)7 = 7C3 * x 4 * y 3

(x + y)7 = 35x4y3

  

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