Effective duration and convexity

Is the total change = -100 effective duration y + 100 convexity y^2?

It’s the duration number and convexity number. Let’s say they are 5 for duration and 2 for convexity a 100 basis point change is. -5 (.01) + 2 (.01)^2= -.0498 Multiple that by 100, you get -4.98% change

Why does the text book multiply convexity by 0.5 when calculating the convexity effect? Whereas my Schweser notes don’t?

Check the CFAI errata, i think they changed this.