Answer:

To substitute means to replace one thing by another. Substitution is the process of replacing the letters in a formula by numbers. For example, the surface area A cm² of a cylinder of radius 5cm and height 12 cm can be calculated by substituting 5 for r and 12 for h in the formula A = 2 π r (r + h)
A = 2 π x 5 x (5 + 12) = 170 π

But substitution can also involve substituting one algebraic expression for another. For example, if a shoe box has dimensions a, b and c and it is required to find the length of the diagonal PQ, then by Pythagoras theorem
PQ² = PR² + RQ²
But PR² = a² + b²
And RQ = c
So substituting for PR and RQ
PQ² = a² + b² + c²
from which