r - Obtain combined standard errors of main and interaction term -

how combined standard error of main , interaction term? in model1 wish combined estimates progacademic , progacademic:math.

i have provided sample data below:

df <- read.csv("http://www.ats.ucla.edu/stat/data/poisson_sim.csv") df <- within(df, { prog <- factor(prog, levels=1:3, labels=c("general", "academic", "vocational")) id <- factor(id) }) mdel1 <- glm(num_awards ~ prog + math + prog*math, family="poisson", data=df)