Question

Find the equation of the tangent line to the curve:
y = ( 1 + 2x)
at the point (2, 25).

Answer 1

the first answer y = 20x - 15

Explanation:

since the correct tangent line must share the same point with the originated function, only a line function that delivers y = 25 for x = 2 can be correct.

only the first answer option provides this (20×2 - 15 = 25).

so, we know, this line goes through the given point, but does it have the correct slope for the tangent ?

the correct slope we get from the first derivative of the original function.

f(x) = (1 + 2x)² = 1 + 4x + 4x²

f'(x) = 4 + 8x

for x = 2 we get the tangent slope at that point as

4 + 8×2 = 20

and the provided line is again

y = 20x - 15

with the slope being the factor of x (20).

so, it fits, and indeed, answer 1 is correct.