Why do I get Error using * Inner matrix dimensions must agree.

This is my code.

t = [-4: 0.01: 3.99];
A_c = 1;
f_c = 50;
m_t = sinc(2 * (t - 2)) - sinc(2 * (t + 2));
k_a = input('enter the value of k_a: ');
s_t = A_c * (1 + k_a * m_t) * cos(2 * pi * f_c * t);
idx = (t >= -2.5 & t <= -1.5);
hold on 
plot(t(idx), s_t(idx), '.r');

Answer 1

[Image Analyst]

Use"dot star" instead of "star" to do element by element multiplication instead of matrix multiplication:

s_t = A_c * (1 + k_a * m_t) .* cos(2 * pi * f_c * t);

[Porgs]

OK, that gave me this error
Error: File: exercise_1_b.m Line: 8 Column: 28
Unexpected MATLAB operator.
this is my code
t = [-2.5: 0.01: -1.5];
A_c = 1;
f_c = 50;
m_t = sinc(2 * (t - 2)) - sinc(2 * (t + 2));

% b)
k_a = input('enter the value of k_a: ');
s_t = A_c * (1 + k_a * m_t). * cos(2 * pi * f_c * t);
plot(t, s_t);

[Image Analyst]

You didn't do it right. You have no space between the ) and the dot and you have a space after the dot. Do it like I did "space dot star space"

[Porgs]

that worked 
thank you so much