At A Level, you may be asked to integrate parametric equations. Sometimes, the best approach is converting to Cartesian form first. For other cases, there’s a way of integrating the parametric equations without converting.
Transcript
Here’s a reminder of the key points you should know about integrating parametric equations.
If we’re given a curve that’s defined parametrically…[x=t , y=t , 0≤t≤π ]
And asked to integrate y with respect to x…
It might be possible to just convert the parametric equations into cartesian form first.[y=1x ]
And then integrate as normal.[∫y dx=ln x +c](pause for 1 sec)
If we want to avoid converting parametric equations to cartesian form before integrating…[x=3t+4, y=4cos t , 0≤t≤2]
We can just take our equation for y…[highlight y=4cos t ]
And take the integral of both expressions with respect to x…[y dx=4cos t dx]
Then, take our equation for x…[x=3t+4]
Differentiate… [dxdt=3]
And make this the subject, so we can convert “dx”.[dx=3dt, plug it into the integral so we get:y dx=4cos t (3) dt ]
Finally, expand and integrate as normal.
[y dx=12cos t dt]
[y dx=12sin t +c](pause for 1 sec)
And if we now want to find an area under the curve…[between 4 and ]
We could rewrite our integral in terms of x. [t=4x-π12, plug into the result to get y dx=12sin 4x-π12 ]
…And evaluate it between the boundaries of our area. [4y dx=[12sin 4x-π12 ]4=62](pause for 1 sec)
However, the smartest thing to do will almost definitely be to convert the boundaries [highlight 4 and ] into t values instead. [take the formula t=4x-π12 and replace the boundaries with t=4(4)-π12 and t=4()-π12, and morph into 0 and 4]